I have just downloaded mac os High Sierra version 10.13.1 and want to uninstall it

MacOS does not allow an older version to be installed on top of a newer version.

ONLY if you have a Trusted Backup, you can erase your drive, which deletes ALL your files, and re-install another version, then restore you Applications and files from your Trusted Backup. It would be even better to have a second copy in case something goes wrong with the first copy.

If you do not have a Trusted Backup, DO NOT erase your drive. You would need a blank drive, such as an External drive, on which to Install whatever version of Mac OS you wish, then migrate your Applications and files.

Or you could ask Readers here to help you with the problems you are encountering.

Why do you want to go back....?

You can not simple uninstall it. To go back, you must completely erase your startup disk, install the previous macOS version and then migrate or restore from a backup of the previous macOS version.

I have a significant number of personal, financial, historical and inventory files on FileMaker Pro 11. After upgrading to High Sierra, FMP 11 no longer works and crashes when starting up. I'm stuck. I can't open the password protected FileMaker Pro file with all my usernames and passwords. Unfortunately I didn't check out FileMaker Pro 11 compatibility with High Sierra - my mistake. I doubt Apple will be concerned about 3rd party app conflicts and there will be no fix for FileMaker Pro (even though it is a subsidiary of Apple). The High Sierra compatible version of FileMaker Pro will cost $325+, more than I want to spend on a fixed retirement income. I did a SuperDuper clone and a TimeMachine backup of my MBP on an external drive before upgrading to High Sierra. Can I uninstall and reinstall High Sierra from one of these backups? How? Thank you!