You are replying to a post which is three years old and so much has changed since then. So really nothing else in this thread relates to your particular question. (in the future, look for threads which have had activity in the last 12 months, and that way we don’t keep resurfacing really old stuff which is just going to confuse everybody.)
Anyway, there should be at least three different ways to move them.
- long press on the device name and you should get a pop-up that lets you move it
- short press the device and it will bring up at details page. Tap on the three vertical dots in the top right to open the edit menu and that’s where you can edit the details including what room it’s in
Also, the + is for adding a new device, not for editing an existing device.
- Instead, click on the three horizontal line icon on the upper left, then choose devices and you should see your devices there. Select one, and you will be at its details page and you can use that 3 dot icon to change the room
So that’s three different methods, hopefully one of them will work for you. 
If those don’t work for you, please start a new thread and ask your question and people can help you. You can use the following section of the forum:
https://community.smartthings.com/c/smartthings-clients/smartthings-samsung-connect/