The 'SWAP MODE' allows you to exchange the positions of adjacent branches of the displayed tree.
Click on the ninth button in the left column of the 'ARB_NT' window to activate the 'SWAP MODE'.
Move the cursor to any position along the edge connecting the two branches and the rest of the tree (with respect to the indicated [open rectangle] root) and click the left mouse button.
The topology within the subtrees is not swapped.
Use the right mouse button, to flip the whole subtree.