Help with wiring for 240 volt heater

When I had my garage built a couple years ago I asked the electrician to put in one 240 volt plug so that I could run a construction heater sometime down the line. I didn't have one at the time so I didn't really pay attention to the outlet he installed. I finally bought a 240 heater and now I'm realizing the plug he installed won't work for what I need.

Here's the heater info:




Here's the breaker box (he labelled it as 240 volt):





Here's the outlet. 30A 250V is stamped on the front:





What will it take to make this work? Is there more to it than just replacing the outlet?

 

Two step process.

1. Change the breaker to a 20a (240v dual bar) breaker.

2. Change the plug to be a 6-20p plug.

This will leave the white neutral wire in the outlet box unused. That's OK. Just cap it off with a wire nut but leave it there for future use.

EDIT:
Main safety precaution while you're swapping out the breaker is to turn off the main panel breaker switch that provides power to this subpanel to kill the power to the bars in this sub-panel.

 

Excellent! Thanks.

 

When you go shopping for parts - take the existing breaker with you and find the identical one that's 20a 240v instead of 30a 240v. There's a befuddling array of very similar breakers on the shelf. I find the only way I can get the right one is to have one I need to match in hand.

 

Good plan. I was going to try to get away with pictures but it'll be easier to have it with me.