You can cut them in half. The recommended dose to help relieve anxiety is 20-40mg, so half of a 40mg tablet would be a good start.
Propranolol is usually tolerated and has few side effect for most people, especially when taking 20mg. You could get a little light headed, slow pulse, or lethargy, but there's a pretty good chance you'll have no side effects.
You could even cut them in quarters (10mg), just to check for side effects, but they may then not provide much benefit for your anxiety. Best wishes.