Possible Duplicate:
Is it possible to have a Game Center “LowestScore” Leaderboard?
My leaderboard contains current amount of gold you have.
Is it possible to force server to update scores even if they are lower than previous?
No. The scores stored for each leaderboard are the highest scores ever reported. When a score is reported that is lower than the value already stored it is ignored.
There is a context field in the GKScore object that you might store the amount of gold in but it will only be saved by Game Center when the score is higher than the previous high, so probably not what you're looking for.
The alternative is to figure out a way to store the amount of gold in a file on the device. The easiest way (programming-wise) is to use [NSUserDefaults standardUserDefaults]. But the defaults file can be edited (changed) by the user so players who know how to do it will give themselves more gold. To avoid cheating you have to encode the value in a way that lets you detect that it was changed, then when the player changes the value you put up an alert that says the game state was corrupted and reset the game to the beginning. The trouble of doing all that balances out the simplicity of NSUserDefaults. There are other ways to store the value on the device that would make it hard to change but they are also harder to learn.