Here are the steps you need to foloow:
1)Unregister the location that is currently associated with the module that has the mappings
2)Edit the location information in the designcenter and change it to reflect the new values
3)Register the location again with the control center and redeploy the maps
you don't usually change db user, you change the password, if you change users, then all the related mappings have to be "relink" to the new location either new created or modified like above and redeployed.