This mod allows to change your skin in-game. It works on client and server-side. You need to upload skins to the mod file, otherwise nothing will work.

Updates

!!PROJECT REVIVED!!

PLEASE WAIT FOR THE UPDATE!

that's it.

Instructions

Open the mod file (using any archiver, like WinRAR) and go to assets/playerskinchanger/textures/entities.
Rename the skins to skin(number).png (without brackets and spaces).
Upload your skins there and that's it!
You can launch Minecraft. To change skins, use the command /skinmanage set (skin number) [toggled] [3D] [Slim]. (values in () are integers, and values in [] are boolean, or true/false. Without brackets.)
I understand that it's inconvenient, but this will be fixed in future updates!
The mod also has a system for automatically setting up the skin when entering a world.
Go to .minecraft/config and find the file skinAutoset.toml there.
In this file, set everything to your taste and that's it!
Enjoy :D

Changelog

25.08.21, v1.0.1 - Added arrows to easily change skins and made preview easier

In-Game Player Skin Changer