Your MUST first make sure your seat's Rigid LATCH (UAS) connectors are completely extended, as the red Rigid LATCH (UAS) Release Handles will only function once these connectors are FULLY extended.
Release Rigid LATCH (UAS) Connectors
Pull black Rigid LATCH Adjustment Handle in between the two red corners and pull the entire seat forward until both Rigid LATCH (UAS) connectors stop extending.
NOTE: It may help to shimmy the seat from the side to side as you pull the seat forward to ensure both connectors are evenly and completely extended.
Once extended, pull the red Release Handles one at a time to disengage each connector.
NOTE: Please see this article to assist you if experiencing difficulties releasing the connectors.