i have an open ground..
 

how can i fix an open ground on my plug

Plug or outlet ? The two are complete opposites.
However, either way you have to attach a ground wire to the ground prong/lug of the plug/outlet and attach the other end of the wire to an approved ground source (another ground wire close by or to the ground buss bar in the breaker panel) or for a plug, just make sure the ground wire is attached at both ends of the cord.

Find the open connection and repair it.

Put a two-prong receptacle back in there like it used to be.
Or install a GFI receptacle for added saefty.