How can I increase or reduce the number of times a liner airs?

This can be done by rescheduling your liners.

If you are scheduling liners using the "Schedule liners immediately as entered" option, you can reschedule by clicking on a liner from the Home Calendar > Liners tab, and then in the popover from the liner grid, click on the Reschedule (backwards clock) icon. This will allow you to change the number or liners and run dates, and then all associated liners will be removed and rescheduled.

If you are scheduling liners using Priority-based Liner Scheduled (daily), first change the number that you want to air the liner each day or daypart and then simply choosing a day that you have already scheduled and scheduling liners on that day will result in rescheduling all of the liners.