Typing "PartOf" in Excel changes automatically to part of

Aside from switching it off permanently for all future, as described in the other answer:
if you want it only once not done, you can type CTRL+Z (undo) right after the change happens - this will undo the 'correction', not your typing.

You can turn it off or remove that specific change:

Go to Excel Options --> Proofing --> Auto correct Options.

Uncheck the Replace Text as you Type or find that specific line in the list and delete it.