If you upgraded and do not have a license which cover the new version, you can choose Free license. It will not delete any data. Then make a backup.
To uninstall it go to Control panel, Programs and uninstall ManicTime. It should ask you if you want to keep the data, choose to keep it.
Then go to this page and download the version you want to install.
Install the version you downloaded. It should work and your data should be there.
If it is not, open ManicTime, then go to Tools, Backup and restore, Restore to restore the file you backed up.