Note these are the rates that Epic invisibly stores to pass onto payroll systems hence will normally be a code used by this system. These are not the rate values that are used whilst calculating invoices. These are set on a per-project basis in the project's editor.
Open your user preferences and select 'System Settings / Manhour Rates'. This will display the current known rates and whether Epic treats them as 'normal' time or 'overtime':
Note that this preference screen also allows you to enable Epic to record manhour records without there being employee records present - for more see Enter Manhours.