need to convert 220 to 110 for hot tub downsizing
The answer is it will be easy to change fro 240 to 120v.
Your 240v feed uses 2 hot’ and a ground at a minimum possibly a neutral.
Moving the wires in the main panel will be need to install a 120v 15 or 20a receptacle
Remove the 240 v GFCI and install a single pole GFCI. Put the black on the GFCI tie the neutral pigtail to the neutral buss and the white to the neutral point on the breaker, ground to the grounding buss if a main panel the neutral buss can also be used, if there is a red wire put a wire nut on it. 

Depending on the size of the original feeder you may need to pigtail to a #12 wire on the breaker (and at the new receptacle) you may need a new box or mud ring at the tub location to mount your receptacle and pigtails. 

If the box is in the weather a bell box or weather tight box and an extreme use cover or In use cover for the outlet.

I suggested a #12 wire for the new circuit because the few small tubs I have installed receptacles for were 15 &amp; 1 was a 20 amp. If it happens to be 30 amp 120v (I have not seen for a hot tub but possible that would require a #10 awg wire. For the pig tails if needed.