To set this up, you'll need to go to Settings --> Allowances. Open up the allowance that you'd like this rule to apply (say, Vacation). When the module opens up, you can then click on "Edit" and then "New Rule". Select when the rule is valid from (maybe from this year?), then tick the box that says: expires at the end of the year. Once done, click OK and save.
This setting will apply for all users unless you've set it up so that certain users have individual settings (like this guide outlines: How to set individual residual leave expiration).