Extracting

Just open the .mcworld file with an archiver e.g. WinRAR. Then go to the behavior_packs folder and extract your behavior pack. If the behavior pack requires a resource pack, then you will need to extract that too. You can find it in the resource_packs folder.

Installing

To install your behavior pack and/or resource pack, you will need to go to the Minecraft Bedrock directory. Normally, you can find it here

C:Users(username)AppDataLocalPackagesMicrosoft.MinecraftUWP_(a bunch of letters)LocalStategamescom.mojang

Inside that directory, there will be two folders, one called behavior_packs and the other called resource_packs. Put your behavior pack(s) and resource pack(s) in their corresponding folders. Then open/restart Minecraft and now you can use the packs in other worlds.

Go to the world you want (or use 7-zip to see inside the .mcworld file) and then go to behavior_packs/. In there will be the behavior pack folders, just copy the folders in there to your resource_packs folder in your Minecraft Appdata.