Once you've downloaded the image file for your skin (it's typically in .PNG format), head to Minecraft.net and log in to your account. Under the image labeled for Java Edition, click on the link ...

To change your skin in Minecraft Java Edition: Open the launcher. Select Minecraft Java Edition. At the top of the center section, select the 'Skins' tab.

How to change your skin on Minecraft Bedrock. Similar to Java Edition, you can upload a skin you got from the internet, or one you've created yourself for your Bedrock character model.