3 wire 240 double pole breaker to. 110v single pole breaker
You don't need to change the breaker.  Just move the white wire from the breaker to the neutral bar.   The other half of the breaker will be unused.

And change the receptacles to NEMA L5-30 or TT30.

You cannot put normal, common NEMA 5-15 and 5-20 receptacles on a 30A breaker.  In fact, 30A 120V circuits are almost useless, except for small campers using the TT30. 

If you need to put common receptacles on this circuit, you must change the breaker to 20A.  (or 15A if there is only one socket and it's 15A). 

If you need 30A capacity but need to feed devices with standard 15/20A plugs, then you need to fit a 120V-only subpanel, with all the rules for that (e.g. if it's in an outbuilding you'll need a main shutoff and grounding rods). It can then serve 2 or more 15 or 20A circuits. In fact, that is how my house is wired.