Install:
Go in your Battle Realms folder, backup Battle Realms.dat and paste the modded one. 

Step By Step Guide:
1.Go in Steam\steamapps\common\Battle Realms
2. Copy "Battle Realms.dat" and paste it in a new folder named "Original Game".
3. Paste "Battle Realms.dat" from the mod into Steam\steamapps\common\Battle Realms

Uninstall: 
1.Go in Steam\steamapps\common\Battle Realms
2.Copy "Battle Realms.dat" from the "Original Game" folder and paste it in Steam\steamapps\common\Battle Realms
Note: If you are asked to replace the old file, confirm.