Note: The 13th period can't be budgeted
If your fiscal year is set up to use 13 periods, you can't assign a budget to the 13th period. That period is meant to be used for adjusting entries only.
The account you want to update should be displayed in the History and Budgets window. (To find the History and Budgets window)
Idea: Remember how positive and negative numbers affect accounts
Be sure you properly indicate whether the amounts in the Budget column should be positive or negative. "Positive" and "negative" may appear to mean the opposite of what you expect; for example, if you enter $100 for an expense account, you indicate that the account will cost you $100 for the month.