BrowEdit allows you to open multiple map instances. To cycle between each instance you use "tab". The Copy/Paste function provided by the Global Height Edit tool can be very unpredictable resulting in irregular editor crashes. You can highlight an area in a map followed by clicking the letter 'c' to copy that portion followed by hitting 'tab' and then left clicking to paste the copied piece. The annoying thing about this is that GAT and terrain height placements do NOT scale with the copied piece onto the paste area.
What I mean by this is if a copied piece from a map is some X units high, than wherever you paste it will also result in the same X units high. So if this new area is X units lower than the copied piece, the paste wont scale. Another thing to notice is if you include lightmaps in the copy selection parameters, you will have to use the clearmap tool -> select lightmaps on the map you pasted the copied area to. This is because of the different map dimensions and what the copied lightmaps were originally scaled to.
So what I'm to get to is if you are unfamiliar with using the copy/paste tool, I would advise you to stay away from it until you practice using it in test case maps where you get comfortable with it and it works/reacts. You don't want to risk ruining your mapping project because you were testing out the copy/paste tool. Therefore stick with changing the Prontera square manually with the provided edit modes.