Depress the button on the seat belt buckle.
If the belt does not fully retract, pull it out and check for kinks or twists. Then make sure it remains untwisted as it retracts.

NOTE If a belt does not fully retract, inspect it for kinks and twists. If it is still not retracting properly, have it inspected at an Authorized Mazda Dealer.
Fastening the Seat Belt
Front Shoulder Belt AdjusterSeat Weight Sensor Inspection [Two Step Deployment Control System]
CAUTION:
If any of the following work is performed, perform the seat weight sensor
inspection using the M-MDS.
Removal of the passenger-side seat
Loosening and retightening of passenger’s seat fixing bolts
Or, the vehicle is involved in a collision
...
Seat Weight Sensor Calibration [Two Step Deployment Control System]
CAUTION:
If any of the following work is performed, perform the seat weight sensor
calibration using the M-MDS.
Replacement with a new seat weight sensor
Replacement with a new seat weight sensor control module
Replacement with new passenger-side seat pa ...
Power Brake Unit Removal/Installation [Mzr 2.3 Disi Turbo]
CAUTION:
Once the brake switch clearance has automatically been adjusted, it cannot
be adjusted again. Therefore, replace the switch with a new one when replacing
the power brake unit or performing any procedure that changes the pedal stroke.
Secure the steering wheel using tap ...