First, be sure the firearm is unloaded before attempting any service. To remove the TG949A/B fiber-optic insert, you need to press down a bit on the middle of the fiber insert, this in turn will depress a small rubber disc under the fiber. This rubber disc applies upward pressure on the fiber-optic, bowing it slightly, and this tension keeps the fiber locked into the retention apertures.
Then, as you press down slightly on the fiber-optic, you will simultaneously push the fiber to the rear of the firearm to remove. To reinsert, reverse the process with the larger flared end facing towards the rear of the firearm.