Go to the official Minecraft website and sign in with your Microsoft account
Open your Minecraft Java Edition profile
Find the skin section
Choose a skin model: Classic or Slim
Upload a new skin file in PNG format
Select the file from your device
Save or confirm the change
Open Minecraft Java Edition
Log in with the same account
Check that the new skin appears in-game
If needed, restart the game or wait a few minutes for the skin to update