There are several ways to do this but probably the most direct would be to double-click on the dimension that you want to be "driven" (blue) by the "driving" (red) dimension. When you double-click on the dimension, you will get a menu at the top where you can rename, apply a formula, comment, etc. Click on the value of the "driving" dimension and it will make them equal..
If you go to Tools-Variables, you'll get more details on "stuff" you can do with all the variables in your model.
Hope this helps,
Kevin Grayson Solid Edge Technical Consultant email@example.com