Of course you can tune to B. If you can change tuning to B just by flipping levers or not depends on what key you have it tuned to when the levers are not engaged.
If you normally have your harp tuned to C without the levers engaged then tuning to B is easy as raising the F, C, G, D and A levers to sharp those notes.
If you are tuned to, say, Eb you would first need to manually return the As to natural with the levers disengaged. Then raise the F,C,G,D and A levers to make those strings sharp, and also raise the E and B levers to make those strings natural.
If you don’t have levers on all the strings then manually retune those strings to sharp or natural as needed with your tuning key, and then tune them back down when you are done.