How to restore or revert to an NMM backup
The new NMM version 0.60.0 will support mod profiles, this will change the way mods are installed on your hard-drives and in the game's folder. Should you not like the new functionalities a backup file will be automatically created for you by NMM, the "vanilla" NMM version 0.54.0 (and above) will be able to read this file and restore your previous mod install.
The Backup file
The backup file is automatically named Backup_DONOTDELETE.zip, it will be placed in the selected VirtualInstall folder (usually in your Mods folder for that specific game), check the picture for reference.
How to properly restore the backup
If you installed any mod using the Profiles version of NMM, or you just allowed it to migrate your mod install, it's better if you uninstall all mods using this version: go in the Tools menu (check the picture), and select Uninstall all active mods , this will automatically and properly clean your mod installation and any file link created by this build, you can now close and uninstall this NMM version.
Open your NMM version 0.54.0 (or higher), go in the Tools menu and select Load Backup (check the picture for reference), find and select the Backup_DONOTDELETE.zip file, confirm, and NMM will automatically restore your previous mod install (keep in mind that you will be required to interact with scripted installers, like the one used by the mod "A Quality World Map" for Skyrim), this procedure will take several minutes or even hours based on the number and size of mods, and the performances of your system .