Look on the sides and you'll see the nails which hold it on. Grab them with good pliers or vise-grips and twist & pull those out. If the heads separate then push the shaft through with a nail-set. Do NOT try to pull these like a nail in a board or you'll bend the metal beyond repair. Once the metal is off, lay it face-down ob a soft board and gently bang it back into shape. Better to stop short of perfect as a slight dent looks better than a bulge. Put it back the same way with trim nails made for the job, then caulk as needed.
Often siding installers will leave the metal a bit loose, or will use a straight bend instead of following a contour underneath. It looks better that way but it leaves it vulnerable to denting. Bondo doesn't work well here because of the metal being too thin and flexibile. If you ever need a replacement piece, look for a crew doing a siding job. Bring them your old piece (no matter how crumpled) and they'll bend you a new one for a few bucks- bum some nails while you're there. If you call a siding contractor to do this they will charge dozens of dollars to do the same thing due to the time needed to set up the metal brake and stow it afterward.
Phil