Assess the damage. A board may be broken but there may also be weaknesses other places in your structure. Check the frame and base of the bleachers. If the frame is also made out of wood it could be suffering from weather damage.
Remove the broken boards and sections of the frame that need replacing. Use a hammer if necessary.
Measure the length and width of the wood that needs replacing. Most of the boards used for seating are 2 by 10s. If a board is unmeasurable or there is no board to measure then the length of the board should be approximately a foot longer than the bleacher frame.
Cut the new boards to the appropriate length.
Screw the new boards into place using a drill and deck screws. Drill from top to bottom so the point of the screw will not injure anyone. Make sure your bleachers are sturdy throughout this process.
Sand the wood so no one will get splinters.