If you want to make it snow manually, you must first activate cheats in that particular world so that you can use commands in Minecraft. If you are in a survival world, head to the pause menu ...

This is possible to a degree by using in-game commands made available by enabling cheats. However, this command won't necessarily change a biome's blocks, but an area's biome classification.

Players interested in automating their commands in Minecraft will have to use Command Blocks, but they’ll need to update their settings to add them into the game.

Minecraft's command blocks allow players to automate console commands, but they are hard to find, so this guide shows how to get them.