I want to play on this server called SkyBlockMc, but it runs on (Bedrock) 1.12, and I'm on 1.13. How (if possible) can I join the server?
I dont know if the option is on mobile but on PC you can request to run another version of Minecraft.
If it isn't there; well you don't have much option else but to ask the server admins to update their server to work with your version and wait for them to do.
EDIT: For @brododragon, let me explain the point. I'm no Minecraft server expert but here's how I understand it.
A game server will expect the connecting game client to have a particular game version (or maybe range of versions, if possible). That could be seen in early PUBG days, where if you tried to login too soon before or after an update, your client would be a different version than what the server expected, and it wouldn't let you log in.
Why is that? Well, if you are a couple versions in front of the server version, you could have/craft some items that are not in the server registry, and if you are a couple versions behind the server version, well the server could make you loot items/encounter mobs that you have no data on. Both would probably make your client or the server crash, and would at least be such an hassle that it would just be easier to force everybody on the same version.
So with the fact that the server is expecting the client to be a particular version number on the table, the asker now has 2 options in front of him.
A- Force/Update his client to conform to the expected version of Minecraft and its mods
B- Request the server admins so the Minecraft/Mods versions of the server confirm to his client.
You either have to download and use an old version of Minecraft, or petition the server admin to update their version.