Go to the 'Topology F1' tab on the top right.
look down to the Advanced tab, then one line above, you'll see a rectangle with an arrow pointing down. That adds baseboards.
So further right you'll see a rectangle with an arrow up, ... that one deletes baseboards. Click on it first, then point your mouse on the relative baseboard and click again.
Once it's deleted, you can't get it back. Sometimes it's a good idea to save a duplicate file of your route first just in case, you click the wrong one.
Cheers, Roy