Not sure what exactly you want to do, some changes are possible, others will require checking the source code as well.

Example:

• You want to add extensible fields so that the IDF Editor doesn't crash if you have for example a ZoneList object with more than one hundred zones: that will work just fine
• If you want to change the mix/max values accepted for a numeric field: this might not work. The source code might have internal checks to make sure your don't put out of bounds values. But it's worth a shot.

Anyways, to edit the .idd file:

1. Locate the .idd file at the root of your EnergyPlus folder. Typically C:\EnergyPlusV8-5-0\Energy+.idd on windows
2. Make a backup copy (just copy paste in place and rename to Energy+-old.idd) in case you make a mistake and cannot figure out how to revert back.
3. Open it with your favorite a text editor
4. Make your changes. Look closely at the structure of the .idd file for how to change it, it's pretty self explanatory^[1]
5. Save

^[1] In the .idd file itself as well as the start of the Input/Output Reference guide at chapter IDD Conventions you have some explanation of the fields. I suggest you read that too.