For the Optifine you already have, I'm not sure on how to change a pre-existing cape unfortunately. You could search through the game settings that Optifine adds to the game for something cape or player model-related. Alternatively you could check if the actual mod files on your computer for Optifine might have a file that determines what cape your player uses.

If that doesn't work you could also download Forge (if you don't already have it, it's basically a passage for downloading mods). Then you'd have to download a capes mod (i.e MCCapes but idk how many versions it covers).

Check out some sites to get full instructions though--this is just what I happened to remember. I'm sure there's a way to change the cape you currently have within Optifine. Hope that helps check_circle