in WHM Search MX and select domain
This was changed in WHM/cPanel version 64, it was renamed to Email Routing, though you can no longer edit the MX record in this area.
Here is a quote from cPanel:
MX Entry interface now the Email Routing interface
In cPanel & WHM version 64, we renamed the MX Entry interface (Home >> Email >> MX Entry). This interface became cPanel’s Email Routing interface (Home >> Email >> Email Routing). We streamlined the interface and moved the functions to manage Mail Exchange (MX) records to cPanel’s Zone Editor interface (Home >> Domains >> Zone Editor).