How can I Modify a Meshing Region in STAAD Foundation Advanced ? 

If you have defined a meshing region and have identified it as a boundary from within Meshing Setup, you can select the boundary name from within the Data Input Pane on the right hand side and then press Delete Selected Region button, available at the bottom of that window, to delete it. Alternately after you select the boundary name, you can also press the DEL key on the keyboard to delete it. To edit the boundary, simply select the boundary name from within the Data Input Pane and press the Edit Selected Region button and you will be presented with a table in which you can edit the coordinates of the corners.

If you have defined a meshing region but NOT identified it as a boundary, you may select it graphically. Once it is highlighted ( will turn red from cyan ) press the DEL key on the keyboard to delete it. There are no options to edit it as such. You may delete the current one and recreate a new one.