It is recommended to backup your Mantis database on a regular basis. This is easy to accomplish using the mysqldump command:
mysqldump -u<username> -p<password> <database name> > <output file>
To restore a backup you will need to have a clean database. Then run:
mysql -u<username> -p<password> <database name> < <input file>
You can also perform both of these tasks using phpMyAdmin
A good idea is to make a backup script and run it regularly through cron or a task scheduler (for Windows see WinCron). Using the current date in the filename can prevent overwriting and make cataloguing easier.
!!! Backups should always be performed before an upgrade !!!
Make sure to backup Mantis code (which includes your configs + possibly customization), bug attachments / project documents, and database contents.