"How do I find out what it is"
Sorry, all out of memory charms, perhaps use a keychain or password manager?
or change it?
Depends, if you are talking about your filemaker server admin console password, you can change that; but a password for a file itself is not resettable without the old one.
There is a 3rd party password cracker out there. It runs between $30-40. It will not retrieve your password, but literally damage your file in order to insert a new hash into the password string for your account. Note that since this damages the file, it's not recommended to even use it. If your file is encrypted-at-rest, then there is absolutely no way to crack the file.