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If you want to perform a simulation that spans multiple years, you can still use the RunPeriod object and enter a value >1 on the "Number of Times Runperiod to be Repeated" input field. If you scroll down at the link, you will see a multi-year simulation example. EnergyPlus will reuse the same daily weather file data from a weather station's annual .epw file and doesn't require any modification there.

The RunPeriod:CustomRange object is intended to be used with a customized weather file where the data period is different from one year (longer OR shorter). This is usually preferred if you are trying to calibrate a model to utility bills or if you have your own weather data you'd prefer to use instead of "typical average year" weather data. The customized weather file still has to follow the same .epw file format.

If you want to perform a simulation that spans multiple years, you can still use the RunPeriod object and enter a value >1 on the "Number of Times Runperiod to be Repeated" input field. If you scroll down at the link, you will see a multi-year simulation example. EnergyPlus will reuse the same daily weather file data from a weather station's annual .epw file and doesn't require any modification there.

The RunPeriod:CustomRange object is intended to be used with a customized weather file where the data period is different from one year (longer OR shorter). This is usually preferred if you are trying to calibrate a model to utility bills or if you have your own weather data you'd prefer to use instead of "typical average year" weather data. The customized weather file still has to follow the same .epw file format.

If you want to perform a simulation that spans multiple years, you can still use the RunPeriod object and enter a value >1 on the "Number of Times Runperiod to be Repeated" input field. If you scroll down at the link, you will see a multi-year simulation example. EnergyPlus will reuse the same daily weather file data from a weather station's annual .epw file and doesn't require any modification there.

The RunPeriod:CustomRange object is intended to be used with a customized weather file where the data period is different from one year (longer OR shorter). This is usually preferred if you are trying to calibrate a model to utility bills or if you have your own weather data you'd prefer to use instead of "typical average year" weather data. The customized weather file still has to follow the same .epw file format.

-- UPDATE 1 --

To modify a .epw file, I highly recommend using Elements. However, Elements currently does not support weather files with more than one year of data. Therefore, to extend a .epw file to have more than one year of data I would recommend changing the weather file extension to .csv so that you can manually add more hours of data the old-fashioned way in a spreadsheet editor like Microsoft Excel. Once finished, change the weather file extension back to .epw and you should be able to use it for EnergyPlus simulations.

If you want to perform a simulation that spans multiple years, you can still use the RunPeriod object and enter a value >1 on the "Number of Times Runperiod to be Repeated" input field. If you scroll down at the link, you will see a multi-year simulation example. EnergyPlus will reuse the same daily weather file data from a weather station's annual .epw file and doesn't require any modification there.

The RunPeriod:CustomRange object is intended to be used with a customized weather file where the data period is different from one year (longer OR shorter). This is usually preferred if you are trying to calibrate a model to utility bills or if you have your own weather data you'd prefer to use instead of "typical average year" weather data. The customized weather file still has to follow the same .epw file format.

-- UPDATE 1 --

To modify a .epw file, I highly recommend using Elements. However, Elements currently does not support weather files with more than one year of data. Therefore, to extend a .epw file to have more than one year of data I would recommend changing the weather file extension to .csv so that you can manually add more hours of data the old-fashioned way in a spreadsheet editor like Microsoft Excel. Once finished, change the weather file extension back to .epw and you should be able to use it for EnergyPlus simulations.

-- UPDATE 2 --

As @Lefort-Antoine pointed out in their comment below, EnergyPlus did add the same Begin Year and End Year input fields from the RunPeriod:CustomRange object to the RunPeriod object in version 9.0. Since these enable multi-year simulations, the Number of Times Runperiod to be Repeated and related input fields for multi-year simulations were removed. HOWEVER, multi-year EPW files should still be used with removed from the RunPeriod object and the RunPeriod:CustomRange object and not the RunPeriod object.