I think what you mean is you don't know how to erase it?
Basically, you have to go to the room you have the walkable area, click "show this rooms" and scroll it to "walkable areas". You'll notice in the properties, all of the walkable areas have a property 1, 2, 3, 4 etc. If you click that drop down menu and set it to "walkable ID area 0" that will put you in eraser mode. then you just drag the square like you are creating a walkable area....but instead of creating it, it will erase what currently exists in that space.
I thought this was an important topic, so I chose to bump it since I had the same problem in the past.
Basically, you have to go to the room you have the walkable area, click "show this rooms" and scroll it to "walkable areas". You'll notice in the properties, all of the walkable areas have a property 1, 2, 3, 4 etc. If you click that drop down menu and set it to "walkable ID area 0" that will put you in eraser mode. then you just drag the square like you are creating a walkable area....but instead of creating it, it will erase what currently exists in that space.
I thought this was an important topic, so I chose to bump it since I had the same problem in the past.