Not sure what exactly you want to achieve here so am assuming you want to change board members' roles to that of observers?
Note that users can be a Member of a board, or of the Workspace; the latter does not automatically add them to specific boards. When a user is added to a board they're a Member of that board. If someone is a member of a board but not of the Workspace, they are Guests.
Only Premium Workspaces can have Observers to boards; the observer role does not exist in the space.
Add a person to a board
Add an observer to a board
If you do have Premium, also check the space settings to make sure it is set to allow guests in the first place: Set up a Premium Workspace. Either you or an other admin could have limited who can be added to boards, eg. based on email domain for instance.
As for how; it's not enough to tap the user icon, you have to go into the Share popup through the button in the Board Menu Bar to do that, scroll to the user and adjust their role. Also, if they weren't added yet, you might have to add them as a member first.
Hope this clarifies, cheers!
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