friend of mine migrated thier mojang account to microsoft,now opon attempting to change skin it gives a 401 error,inticating that the credentials are invalid,if anyone has experience this issue too,or knows a solution feel free to share them,have a good day.

Image related.

Try using the latest version (0.1.1a) if it still doesn't work use your vanilla launcher to upload the skin for the first time, then it should work.