Open Minecraft and go to > Resource Packs > Open Resource Pack Folder . Move the downloaded file into this folder. Activate it from the in-game menu.
There is an Ultimate Skybox Pack featuring over 3,700 skyboxes with updated shaders. It is available as a free community resource on the Quake3World forums. Download ultimate sky pack rar
For Fabric users, the Fabric SkyBoxes mod is needed to support custom sky textures. Manual Installation: Download your .rar or .zip pack. Open Minecraft and go to > Resource Packs