In the Interior screen you can delete the inner walls of your plan. Just touch the wall and click the Delete / Undelete button on the toolbar.
If you need to remove an outer wall, you can insert a doorway or double doorway into the outer wall and increase it until it's the size of the wall.
If you need to remove part of the wall, go to the Layout screen and split the wall into a few walls (See this FAQ here on How can I edit shape of the rooms? )
Then, switch to the Interior screen, highlight part of the wall, and remove it.
See this video tutorial: Interior Design for iPad - Removing walls