Use the soldering iron to desolder the U.FL antenna connector. You might need to use a desoldering braid wire to remove the excess of solder.

Soldering iron will be very hot.

Clean the PCB pads of any excess of solder with the soldering braid wire before continuing.

Place the new U.FL connector on the pads where the desoldered connector was. Make sure the orientation of the connector is the same as before.

Solder the new U.FL connector to the PCB. Avoid exessive heat and/or prolongated soldering time during this step.