Those stains are normal mineral deposits from your water. They won't affect operation, but you can clean the tank easily.
- First, press the "PRESS TO OPEN" button and lower the handle completely. The water fill lid will pop open.
- Inspect the hot water tank area. Use a damp, lint-free cloth to wipe it out as necessary.
- Close the water fill lid when you're finished.
If the stains are heavy white deposits: That's scale, and it's time to descale the brewer. You'll need 8 oz of white vinegar. Fill the tank to the FILL LEVEL with vinegar, run a brew cycle without a K-Cup into a ceramic mug, let it sit for 30 minutes, then flush with several tanks of fresh bottled water.
Heads up: Only use water in the hot water tank. Do not put any other liquids or foods in there. Always unplug the brewer before cleaning.