Make a copy of your shell start up file before editing it to change environment or shell variables.
You can always return to using this copy if you find that the modified file is not working and you are unable to fix the problem. For example:
cd cp .profile old_profile ue .profile
The first command changes you to your home directory, where a copy of the file .profile is made in old_profile. You can then edit the .profile.
To revert to using the original .profile you simply copy it back. For example:
cp old_profile .profile