To change the angle between walls, follow these steps:
Hover over the corner formed, and select the large dot that appears
In the Corner properties that open on the left, navigate to Change Angle
Select between which two walls you wish to change the angle, by select in from the Angle Between dropdown menu.
Determine which wall will remain fixed by selecting from the Fixed Wall dropdown value.
Type in the desired angular value (degrees) in the Angle (in degree) field.