There are two ways to trim/extend two non-parallel walls.
To make them intersect and create a clean corner,
Select the Trim/Extend dropdown under the Architecture tab and select Trim/Extend to Corner.
Click on the wall that is to be intersected, on the side that is not meant to be trimmed out during the corner formation.
Click on the intersecting wall that is going to be extended to the first.
A clean corner is formed, with the unclicked side of the first wall being trimmed out.
To extend one wall to meet the other,
Select the Trim/Extend dropdown under the Architecture tab and select Trim/Extend Single Element.
Click on the wall that is to be intersected.
Click on the intersecting wall that is going to be extended to the first.
A joint is formed without any trimming of the first wall.