After 14 days of usage your CGM will expire. When the CGM expires you will receive the message "Your sensor has expired. Please replace with a new one," as can be seen in the screenshot below. You can also monitor how many days you have remaining with the sensor by going to Settings -> Sensor Settings.
After you see the expiration message, you should do the following:
Peel off the CGM by pinching a small part of the adhesive that is attached to your skin. Pull the adhesive up and toward the center of your CGM. The CGM should come off easily.
Wrap the CGM in a tissue, toilet paper or paper towel and dispose of it in the trash. Don't worry, there is no needle in the CGM, so it is safe to throw out.
Put on the new CGM. The process can reviewed here.
Reactive the new CGM. The process can be reviewed here.