There is a plastic tab located on the back of the label maker near the top. The tab has ridges on it to help you identify where to press. Push on this tab to release the back of the label maker.
Gently pull the label maker apart - it will split in half.
Be careful! The electronics inside are very sensitive and easily damaged.
Align and insert a new tape cassette. Make sure the center of the cassette snaps into place. The tape must feed between the tape guides.
If the ribbon has become too loose, use your finger to turn the white wheel in the direction of the arrow on the tape cassette. This will wind up any excess tape.
To test the new cassette, make a new label. Press "Shift" and "Print" to feed the tape, then use the cutting lever on the upper-right corner of the machine.