In order to upload your own skin that can be flipped to via the 'Previous Skin' and 'Next Skin' items, you need to directly edit the resource pack, player_models_rp.

This section might be difficult to understand. These bones can each be rotated on all three axes in increments of 90 degrees, 15 degrees, or 2.5 degrees. The 'Pose ' item opens a menu in which you can choose to rotate any of the player model's bones-its head, its body, its arms, or its legs. Additionally, clicking the 'Rotation ' item lets you rotate the whole player model around its vertical axis. Clicking the 'Position ' item from the home menu of your hotbar interface lets you move a player model in either 2 block, 1/2 block, or 1/16 block increments in any direction. The pack lets you spawn in new player models, change which skin they wear, and pose the player models to your liking.

The 'Exit' item that will appear in your ninth hotbar slot can be used to free your hotbar, and you can run /tag add player_models again at any time to re-enter the interface. This command will put items in your hotbar which will act as the interface for the data pack. When you have joined your world, run /tag add player_models. You will also need to paste player_models_rp into your resourcepacks folder and enable the resource pack. In the save folder of whichever world you want to use this pack in, paste player_models_dp into the world's datapacks folder. A pack for adding and posing player models in Minecraft Java Edition.

More player models fabric