I want to change inside walls of guest bathroom to wood- how?

Before installing the boards you need to finish them, on all 6 sides, with a sealer and probably a coat of paint as well in order to prevent them absorbing moisture and warping. Construction adhesive would be a permanent method of attachment as removal of any of the boards, once applied, will probably destroy the wall boards it is glued to necessitating stripping the wall to the studs and starting over. If you think you might ever want to change the boards you could screw furring strips to the wall studs and then screw or nail the boards to the furring strips...the boards still have to be finished on all sides to prevent warping. The second method will result in a thicker wall covering which will reduce the bathroom size by 2 times the thickness of the boards plus 2 times the furring thickness of approximately 3/4" and be more work than the gluing but has the advantage of being able to be removed with minimal damage and more easily at a later date.

Just don't do it. Gonna be rough to change it out when you tire of it.

tongue and groove flooring. Don't glue it, just nail it on with small brads that way if you tire of it all you have to do is fill the tiny holes with dry wall mud, lightly sand and repaint. 😄