I have this visa and need to get the next stamp as the first 90 days will expire soon. I am in Chiang Mai and was planning to go to Mae Sai, but I'm a little confused as to whether I can go here or do I have to fly out of the country? Advice please. Thanks

You must cross a border, enter another country and return to be given a new 90 day stamp

No need to fly unless that is your choice.

You can do a crossing at Mae Sai to get a new 90 day entry.