If you update from 1.2.0 to 1.2.1 you only need to edit the PHP side.
- Get the PHP.zip from the latest version and unzip it.
- Use Filezilla or any other FTP program and connect to your site.
- Download your config.php file from the site to your local computer (So we can move most of the values to the new config file)
- Open the new config.php file and the old config.php and start move values from the old to the new.
- In Filezilla: Upload the new PHP folder and replace any old files.
- Done.
If you update from an earlier version you need to replace the old GML with the new one. And depending on how you added it this can be a little tricky. But if you kept the WebAPI folders intact in the project you should be able to just remove them and import the new ones. I think most of the old features use the same arguments so they should work as before.