How can the lift force be changed without changing velocity?
The problem is:
  Now I want the aircraft to go to balance state to move with constant velocity.
  So all the four forces should equals to zero thus the lift force should equlas the weight force.
  How Can I decrease the lift force?


This can be done by simultaneously reducing AoA (which in-turn reduce CL) by introducing little down elevator and reducing the engine power accordingly to maintain the airspeed.