This is a video I made on how to upload a Planet Minecraft cape to Minecraft. You need to have a current cape for this to work. For recent Java edition version 1.11+? (you can only do this on java edition in 1.11+, not sure exactly what versions it works on but its worth testing even if it is less then 1.11). This may only work with the migrator cape. Here are the steps:

1. Download the cape you want Testing Example
2. Copy the cape in files
3. Go to .minecraft folder
4. Go to assets folder
5. Go to skins folder
6. Search and go to 17 folder
7. Go through the files in the 17 folder and fine the one with the migrator cape (image of what the cape texture looks like at bottom)
   
8. Paste new cape (do this before copying the name, i made a mistake in the video)
9. Copy name of old cape
10. Replace the name of the new cape with the old cape and remove the old one

That's it!

Image of the migrator cape file

If you have any questions let me know in the comments!