I have done this in the past by simply deleting the existing equation and replacing it. It is the only way I can figure to do it with IEquationMgr.
As long as you know the index of global variable in EquationMgr it is pretty easy to replace. In some cases I simply delete all global variable equations set the new values and then re-write the text strings for the equation mgr.
Thanks Patrick! So simple, yet effective. I am sure it gets more complicated if there are multiple global variables but in my case there is only 1 that drives a few equations so it seems to work seamlessly to use your method.
For reference for anyone else needing help with this method here is the code that I used:
Dim swEqMgr As SldWorks.EquationMgr
swEqMgr = swDoc.GetEquationMgr
swEqMgr.Add2(0, """belt""=" & beltwidthint, True)