Open Minecraft
Go to the main menu
Select the profile or dressing room option
Choose Edit Character or Skin
Select a skin from your owned skins or import a new one
If importing, upload the skin file
Save the changes
Start or rejoin a world to see the skin
On Java Edition, open the Minecraft Launcher
Go to Skins
Click New Skin
Browse and select the skin file
Choose the model type
Save and apply the skin
Launch the game to use it