Watch this video to learn how to turn 90 degree corners with a serger. Also watch at 9:30 about how to finish a seam by basically turning 90 degrees away from the garment and just chaining off. For my serger, I can’t loosen the threads in the way shown in the video. Instead, I raise the presser foot, then I push/yank the garment away from me, which causes the threads to jump off the stitch finger (?). Some people turn each of the tension dials down to 0 to loosen the threads. Once you have the thread loose, then you should have enough room to pivot 90 degrees.