Diagnosis
The electronic filter life monitor is tracking water volume, not just time. If your household water usage is high, or you had a lot of initial flushing, it can hit the volume limit early.
How to Fix It
You need to manually reset the indicator. With the unit powered on (green or red light on), press and hold the 'Filter Reset' button on the front for about 5 seconds until the light cycles and returns to a solid green. That resets the timer for another 6-month/600-gallon cycle.
If That Didn't Work
If the light won't reset or immediately goes red again, the actual filter might be clogged prematurely by unusually bad sediment. Check water flow from the filtered faucet. If it's a trickle, you probably do need to replace the filter early.
Heads Up
The light is just a timer. If you reset it without replacing the filter, you're responsible for your own filter change schedule. Mark your calendar or track gallons manually.