you need to go to settings in Zune 4.8, then under phone settings, click on update. let it search. after it loads, the lower half of the page is the restore section which will tell you the restore date. you need to click on restore.
This is assuming that you went the official way in order to update to mango.
If you used the tools from windowsphonehacker.com (the unofficial way), then your backup will be in C:\premangostate\ which you need to copy to the Zune backup location (C:\Users\username\AppData\Local\Microsoft\Windows Phone Update\ ) and then run the restore from Zune settings.
If you don't have a backup at all, you will need to flash a ROM image to an older version . There are many tutorials for this around xda. I recommend any of ansar's ROMs. You can't use a backup file from another device as backups are phone-specific. Phone A's backup won't work with Phone B.
Or you could just enjoy using preMango and build a few apps which all of us would enjoy using!