How to restore all environment variables to default in a batch script

I have a .bat script. It runs some commands and alters/adds some environment variables. Then, at some point I need to revert them all to default as if I'm exiting this script and starting a new one from scratch.

So, I need one of the two:

1. either clear the variables (restore defaults that a fresh cmd session gets);

2. or push all the variables when I start the script and pop later to revert to these values.

How can I do that?