Tab on the wall you wish to edit. On both ends you will see nodes - you can drag them in any direction to change the wall’s length or position.
You can enter a new length directly into the measurement.
An alternative way to enter an exact measurement, is by using the i-button:
The wall’s Property Inspector will open. Choose from which side of the wall you want to increase or decrease the length. To change the direction, tab on the icon showing a wall with an arrow.
Next to it you can decide whether or not you want to keep a 90° angle when the new wall length is applied to your plan. Bellow are two examples of how the same plan changes when the right angle function is off and on.