Manage wifi 6 channels

How on earth do I change the channels on my modem/router. I login and it says your channels are managed.
I had a tech come out 3 weeks ago and he changed my 5ghz channel to 44 from 112. Since then my wifi speeds dropped 50%. The tech had no knowledge of wifi 6 at all. He stated he had never heard of it but he was here for the alarm. But changing the channel killed my speeds.
I would like to change my settings back for starters.

And I won't go into the trouble I have had with my alarm system and the lack of response from executive support.