This dialog is fairly self explanatory.

You can change the key by cycling through keys with the arrow keys with feedback, you can also use the buttons to change the key to major or minor or if you already know what key the segment needs to be in.

FIXME add a picture! Also someone with more knowledge of transposition might like to review anything I've said on this topic.

There are other options to choose the scope of the change and how the key transposition will be handled.