How To Fix Input Signal Out Of Range Change Settings To 1600x900-60hz

Great tutorial! It is really helpful! I have a question. I made the entire site in Moodle, with the purchased theme. But, how to upgrade Moodle installation safely? I currently have version 3.2, but I would like to upgrade to the latest, 3.5.2. P.s. I did not install any additional plugin, except the theme. Thanks in advance!