not a teacher
Use care when removing cereal from microwave; bowl may be hot.
Here, "care" is a noun as per definition #2 in this link.
So, "use care" means to pay attention to doing something in a way that avoids damage or risk – in this case burning your fingers.
For a convenient pantry pack, press in and pull up along perforation on back of box to remove flap.
I assume these instructions show you how to set up the box so it makes a suitable pack for storing the cereal in your pantry.
Student or Learner