I am accidentally watching 1 Forum and I want to watch another. How do I watch and un-watch Forums?

Betsy, at the bottom of this thread is a bar with two columns of options, written in blue letters - the fifth option is either 'watch' or 'unwatch' this thread. You can add as many threads as you want to your watch list.

That's for watching an individual thread, like this one, in a forum. (Threads can also be watched or unwatched at the top left of the thread)
If you want to watch a Forum, click on Forums on the left side, select the forum and again, on the left side is the option to watch that forum. (Or unwatch)

Help. I am not seeing it.

Betsy, anything in blue or purple letters is a link to a place or an action. The Watch this forum link is at the top right in blue letters, under the banner (flower pictures etc.) after you click on "Ask a Question"

If you're on your phone, it won't show. You need to see it on desktop version.

Ok. Thank you! I'll go to my desk top.

You can get to Forums from your phone too @SpringGreenThumb and @Zencat
When you enlarge that phone screen capture above, to the left of the little home/house icon is a circle with three bars. Click that and it takes you to the blue side bar section. Select Forums and the scroll through the individual forums. Click the one you want to watch, it opens to that Forum page with the list of all the threads. Once you are on that Forum page, click the 3 bar circle again to take you to the blue side bar section and you will now see the option "Watch This Forum". Click it to add and of course, do the same steps to unwatch it.