Leave Profiles generally map to an employee's conditions of employment - how much leave are they entitled to, what their working hours are, do they have any Length of Service entitlements etc. If these conditions change perhaps because they change working hours you could adjust the values in their Leave Profile but that would change the values for all employees with that profile. So the right thing to do is assign them to a new Leave Profile.
To make a change click on the user profile > Options button > Select Change Leave Profile.
In the dialogue, select the new Leave Profile. When the employee is moved, use the Allowance Calculator to update the employee allowance in accordance with their new Leave Profile. A summary of the information relating to the new leave profile is shown in the dialog.