Members can be extended or shortened using this tool in the renderer.
After selecting the members to be extended or shortened, right-click and select "Extend" from the menu that appears. You must then select a reference point graphically. This just allows you to control which ends of the members will move and which ends will stay in place. The form shown below then appears.
The "Mode" option lets you choose between specifying a new length or specifying an extension or reduction. The "Move" option lets you control which ends of the members will be moved. In the "New length" or "Extension" field at the bottom of the form, you can specify the new length or extension (or shortening) as an absolute value or as a percentage of the original member length.