Replace a char in string.

Hi 

I would like to convert at string from z300_z400_MA2 to 300.400.MA, z300_z400_MA2 is a tagname. 

Now I am using this function to remove z from the string. 

StrTrimAll(#CM_Property:CM_Drive->Name, "z")

How can I replace _ wiht . ? I can not find a string function for this. 

Thanks