How do I change which test I take day of?

Wanna switch from US History to Lit

Please has anyone had experience doing this

Google is your friend: https://collegereadiness.collegeboard.org/sat-subject-tests/taking-the-test/test-day#changing-tests

“In general, you can take a different test from the one you signed up for — the test booklet contains all Subject Tests available that day, so just take the ones you want. But there are some exceptions.” The exceptions appear to be languages and any test for which you need accommodations. You can also add a test if you want. Good luck!