Parameters between [ and ] are optional (do not include the [ or ]), replace <biome name> with the name of the biome you want.
All selections are rectangular. X and z are (of course) the x and z coordinate of the "lowest" (smallest / most negative coordinate) corner of the region. Width (along x) and height (along z) move in the positive direction. All commands that use coordinates do not care where you are, and can even be used from the server console.
Display a list of supported biome types. Some of these (like "tundra") do not currently work. If you are having problems setting a particular biome on the list, try a different one to see if it sticks.
Display the biome of the block you are currently standing on.
/biome set [world_name x z width height] <biome name>
Creates a biome "region" in the specified world, with the given size and location. If no world/location/size is given, it defaults to the chunk you are currently standing in (players only.) Use /biome list for possible biome names.
/biome set-selection <biome name>
Creates a new biome region using your current WorldEdit selection. This command is not available from the server console, and you must have WorldEdit installed.
/biome clear [world_name x z width height]
Resets the region specified to its natural biome. If no region is given, it defaults to the current chunk. This will not change existing blocks, only their biome data.
Like /biome clear, but uses your WorldEdit selection for the area. This command is not available from the server console, and you must have WorldEdit installed.
Displays the global biome for the current map.
/biome set-global [-w world_name] <biome name>
Used to set a "global", map-wide biome. When run, all loaded chunks will be converted (this will not change the blocks in the chunks, only their biome), and all newly generated chunks will be of the specified biome. General landscape features will remain the same, only the "decorations" of the chunks and their biome data are changed. Use "/biome set-global none" to disable the current global biome (this will not change existing chunks.) Global biomes can be disabled entirely from the BioMed config. If you wish to set a global biome without affecting existing chunks, manually set it in the BioMed config.yml, and reload.