From Quake Wiki

client console command:

loadsky <basename>

loadsky mtsun_ would load "mtnsun_up.tga" and "mtnsun_rt.tga" and similar names use "" to revert to quake sky, note: this is the same as Quake2 skybox naming)

Sets global skybox for the map for this client (can be stuffed to a client by QC), does not hurt much to repeatedly execute this command. In mods it is preferable to use DP_GFX_SKYBOX to set the skybox for the map.