Hi,
There are two options by which you can change the member length.
1. Select the member and go to Geometry > Stretch Selected Member(s). In the new window, select the proper option and provide the details.
2. You can change the start or end node coordinate. For example, the start and end coordinates of a member is 1(0,0,0) and 2(0,0.4,0) respectively. You can change the end coordinate to 2(0,9,0). This will change the member length. You can change the coordinates from the STAAD input editor (Edit > Edit Input command file) or from the GUI (Geometry < Beam).