Go to Setup > Staff Setup > Staff List
Click on the staff requiring contact update.
Click on Staff Work Periods and Hours.
Click on Edit icon to update contract.
Choose correct contract and click on Save icon.
If one contract is expiring and a new one is required:
Click on Edit icon.
Enter End Date of existing contract.
Click on Save icon.
Click on green ADD button.
Enter new start date (cannot overlap end date of existing contract).
Choose new contract.
Click on Save icon.