Just to follow from Dave.
The most important word in Dave's post is "copy".
Keep a copy of all original inp files out of the box.
Keep a copy of any inp file that works before you edit it.
Editing an inp file is very easy by just following similar usage elsewhere in the file. e.g. Changing decimal places.
However, if you make a mistake, the system just stops. There are no error messages and no indication of what mistakes you have made. This is not a problem but you need to have a working copy that can be reinstated.
Edit and test in small baby steps in case you need to go back to a previous copy.