Within the leave types you can set an expiry date. In case you want leave to never expire so it can be accumulated for the coming years, then you can change this for each type of leave in the section Unused hours. You can choose to have the unused hours:
never to expire, so they can be added onto the next year every year
expire at the end of the year, so that they cannot be taken into the next year
expire within a certain time, so they can still be taken into the next year, but they will expire anyway at a certain moment depending on the time frame.
Pick "never" in this case so hours can always be added onto the next year.
Please note! When you do this to a leave type that has already been appointed to employees for multiple years , changing the setting can lead to budgets increasing because they are activated retroactively. Leave allowances that would expire sooner according to the settings, are not expired anymore. You can solve this by creating a new leave type, appointing it to all employees and ending the old leave type.