Close Internal Corner 

Sheet Metal > Change length > Corner/Mitre > Mode: Close corner, free > Joint: Inner edge, flush

Sheet Metal > Change length > Corner/Mitre > Mode: Close corner, closed > Joint: Inner edge, flush

If you choose the Close corner, free mode, the sheets will be lengthened (or shortened) to the common intersection point of their inner edges.

If you choose the Close corner, closed mode, the bend zones and milling edge zones will also be closed.

Proceed as follows:

1. On each flange, select an edge of the front side or, with a right-click, the longitudinal edges.

A preview will be displayed. Confirm with Apply.

Preview
Left flange identified via longitudinal edge (RMB)
Right flange identified via front edge

After "Apply"

(1) Open corner
(2) Front edge
(3) Closed corner
(4) Top view of closed corner; Joint: Inner edge, flush

Close Corner