As for every update process, you have to backup some data before processing any upgrade:
- backup your database;
- backup your files directory;
- backup your configuration.
GLPI update process is automated. To start it, just go to your GLPI instance URI, or (recommended) use the command line tools.
Once a new version will be installed; you will not be able to use the application until a migration has been done.
Please also note the update process will automatically disable your plugins.
You should not try to restore a database backup on a non empty database (say, a database that has been partially migrated for any reason).
Make sure your database is empty before restoring your backup and try to update, and repeat on fail.