How to repair this metal corner siding?
"Siding Corner Caps"
Or often just tin caps. They should be available. Or you could have some fabricated, or perhaps adapt some intended for other siding materials.

Install by running a thin bead of caulk along the end of each clapboard, slide the top under the upper course and press into the caulk. Nail underneath at each corner with a small corrosion resistant ring shank nail.

Alternately, for a different look, take them all off, trim the clapboards back a bit and install a vertical wooden lapped corner.