While you have a review open, RevMan will save a local backup of it at regular intervals. The default is every 10 minutes, but this can be changed under Files preferences.
If your computer crashes so that your review is not saved successfully, you will be given the option to open the latest backup of the review the next time you open RevMan.
RevMan backup files are ordinary rm5 files with file name 'backupXXXXX.rm5' where XXXXX is a number. The backup files are by default saved in your temporary folder. You can see, or modify, the location under Files preferences.
Backup files in the currently specified backup directory are automatically deleted after 7 days (by default).
If you need to access a backup file, simply open it in RevMan. You may have to look at the date the file was modified to find the right one. Once you have found the file you need, save it with a different file name, or you will risk that it is deleted as part of the removal of old backup files.
See also Backup to Archie.