Once you have clicked the Reduce Faces checkbox, you will almost always see a warning box in the right-hand corner asking you to repair your damaged model.
As you can see in the above pictures the more you decimate this less detail you get — take a close look at the medallion around Buddy Cthulhu's neck — so start with a lower number and work your way up. I would start at around twenty-five percent and move up to thirty-three percent if you feel confident the model will support it. The default is set to fifty percent, but I find that can be too aggressive. There is a lot of trial and error involved here as removing too many triangles will destroy the fine details of your model. Here you will see a slider that allows you to chose the percentage of triangle to reduce the model by, the higher the percentage, the more triangles will be removed. From there you will need to go to the edit tab and click simplify. Once you have chosen which model you want to decimate, you can load it into 3D Builder.
