You need to change this setting at 2 places. Firstly, you need to bulk change for all your existing employees. Then, you need to set your new hire retirement age default value, so that in future all your new hires will be correctly assigned. Please follow the steps below to set these:
Global Change
Go to the Core module, click Setup, Global Change.
Select Retirement Age and fill in the number as eg: 63.
Click Next and bring everyone to the right panel.
Click Confirm.
Personnel Details Default
In the Core module, click Company.
Change the Retirement Age as eg: 63.
Click Save.