To change a staff member's pay rate, go to:

Staff > View Staff > Manage Staff Member > Edit Staff Info > View/Modify/Add Staff Member Pay Rate

From here you can either select to Modify the current pay rate (this option will override the current pay rate, and remove the historical data):

or to Add a new pay rate:

When adding a new pay rate, you can set the date you would like for the new pay rate to become effective (this will override the previous pay rate, but keep the historical data for your reference).