How to get my old minecraft skin back?

You can't if the file got deleted if you need a new skin Google skindex and you can edit/use any skin on the website, sorry about losing your skin.

Sorry, Minecraft doesn't save what skins you used to have.

You may be able to find it on the website you used to get the skin, otherwise, you can't.

In future, be sure to save them to your computer when you want to upload them, then you can upload them again from your computer.