The blinking red light next to the REPLACE label means your filter has reached 0% life and needs to be swapped out. That's the indicator's main job.
First, replace the filter. Wash your hands, take out the new filter, and flush it under cold running tap water for 15 seconds. Line up the groove on the filter with the ridge inside the reservoir and press firmly to get a tight seal. Fill the reservoir with cold tap water, discard the first three pitchers of filtered water (or use it for plants), and you're good to go.
Second, you have to reset the indicator for it to start counting correctly again. Press and hold the STATUS button. If you just installed a standard white filter, hold it for 2 seconds until all lights blink twice and the green STANDARD FILTER light blinks three times on its own. If you installed a blue LONGLAST filter, hold the button for 6 seconds until the green LONGLAST FILTER light blinks three times on its own.
If you accidentally set the wrong mode, just repeat the reset process until you get the correct light sequence for your filter type.