How do I change someones holiday entitlement?

This is a 2 fold process.  Firstly you should update your employees base leave entitlement on the people > employee name > Person (dropdown menu) > Work page and update the Base Leave entitlement towards the bottom of the page.

Leave years are created for each year based on the base leave entitlement that we have just updated so you should also go to people > employee name > Leave (dropdown menu) > Leave years > 'current leave year' > and make a manual adjustment for that year.  You can also make the adjustment in minutes which will be more accurate.