Hi Mike, check out the object repair tutorial for a pretty in depth covering of that:
http://moi3d.com/forum/index.php?webtag=MOI&msg=446.17

Also here's a shorter one focused just on how to untrim:
http://moi3d.com/forum/index.php?webtag=MOI&msg=444.4

You basically need to get the boundary that you want to remove to be made up of all unjoined edges (delete things or use Edit > Separate as necessary) then select all the edges of the boundary you want to remove and push delete to remove that trim boundary and restore the underlying surface there.

If you want to untrim everything on a surface, select one edge and then do Ctrl+A to select all its edges before pushing delete.

There's also a plug-in "UntrimAll" which gives a shortcut for untrimming everything in a whole solid in one step here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=4068.6

- Michael