YTD should have reset to zero automatically. My console has a default display setting that is NOT YTD, rather the amount since the unit was manually reset (or new). YTD can be viewed by using the buttons on the rear to step thru the display options. However, there should be a button to manually reset the display or you can remove the batteries for a bit, although consoles differ in operation. The manual reset will zero everything so you'll lose a couple days of data this year.
Your particular display shows your "All Time" rainfall, which is your rainfall since the unit has been powered on. If you would like it to represent the years total, you will need to reset the unit at the first of the year in order for it to do that.