Daniel,

1 - Select your wall(s)
2 - Do a right-click
3 - Select Modify walls
4 - Enter the new wall height(s)
5 - If you are using multiple levels, you will also have to adjust the Elevation in the level settings by double clicking the Level tab.

Suggestions:
1 - Read the User's guide
2 - Start using the Help pages in the program.

Hans