How to transfer budget from the account that fund available remains to the same accounting in the ne

Hello everyone
I'm working on Budget but I have some problem about transfer budget.
I would like to transfer from the account that still have the fund available to the same account in the next budget year.
Eg.

On the Budget year 2012, this account has the fund available remain.
I already set the Budget year 2013. How to transfer the fund available of this account in Budget year 2012 to Budget year 2013.

Could you please advise?

Thanks,
Ying