Hi, mate! You need to unzip the 1.13-B version on top of a full game installation. The best course of action would be to download the 1.12-C version, unzip it and then put the testing version right next to the stable one. That way you will be able to play with 1.12-C and 1.13-B both.
By the way, welcome to MiniRacingOnline! I hope you have a great time here.