For employees assigned to a fixed Place of work Humaans will automatically assign that Locations default policy (see more on configuring here). If you need to change this policy for any reason you can follow these steps.
Go to the user's Manage time away screen
At the bottom of the page click Show time away history
On their start date you'll see the applied policy. You can click Edit beside this.
Here you can select Assign a specific policy
Choose the policy and click Save
Then the policy will be back dated to their start date