Can I alter an individual frame?

As a total beginner, and someone who struggles to understand anything to do with computers and technology, I am having great difficulty in resolving a problem I am having with Harmony Advanced 12, on an iMac OSX 10.11.2.

I have attached images of the first three frames in a project. As you can see, in frame 2 the trouser line breaks up at the knee joint (and this problem is going to keep ocurring as the character moves through the frames). I dont know how to insert a corrected drawing at an individual frame, without changing every other frame in the same way. If I re-paint the frame with the dis-connected trouser line at the knee, I find that it is repeated in every other frame!
Can anyone tell me how to alter/re-paint just one frame, without changing all the other frames?

Thanks for any advice.

Top Toolbar / Drawing / Duplicate Drawings (shift+option+d)

Top Toolbar / Drawing / Duplicate Drawings (shift+option+d)

Thanks nolanscott. - Very useful information.