Here's some information on it:-
The update includes three unlockable weapons—such as the previously unveiled Axtinguisher—which can be obtained via 35 new class-specific achievements. Valve has previously stated that the Pyro update will also include "significant changes" to the core class which will be enacted regardless of whether players have the new weapons.
In addition to the new weapons and achievements, two community-created maps for the game will be included in the update. To tide players over til Thursday, Valve also plans to release Meet the Sniper, the latest video in its series of comedic class profiles, tomorrow morning.
The developer also revealed that it will hold another Team Fortress 2 free weekend, allowing players who haven't yet purchased the title to play the game for free from Friday morning through Sunday evening. During that time, players can download and play Team Fortress 2 from Valve's downloadable PC gaming platform Steam free of charge.