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.
First, download latest GLPI version and extract files. GLPI update process is then 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.