How to crearte biome?

if you're using the value biomes modifier, simply change the slider to the other biome
@kilobit

@Zenithspeed How do I make it appear on the surface? I do it and the biome does not appear, what do I do

you can press "add biome" in biomes tab (rocky circle thing), you then just need to give it a name and it's basically done, you can then add sub biomes and stuff
as for adding the biomes to planet, you can either add your new biome using "add biome" modifier or by adjusting slider in biomes modifier