The best and simplest way I can suggest would be to (assuming you read the instructions properly beforehand and did exactly as it and the website suggest... run a full backup before upgrading):
Uninstall Plesk 7.5
Reinstall 7.1.6 from the RPMs
Restore Plesk backup from the backups you ran prior to the upgrade
psarestore is very slow at restoring server dumps so I would only recommend you do that if you *absolutely* cannot live with Plesk 7.5.0 - if you're having problems with it then (imho) the best solution would be to either report the bugs to SWsoft and await a fix or look for external help.