Hi Audrey!
Income and expense accounts will automatically set back to zero with each fiscal year you create. This is the same with any personal draws and advances if you are a sole proprietor. Simply go to Setup > Fiscal Years, and click on Add Fiscal Year. Follow through on this process. This creates your next year. Close the fiscal years screen, and if you check your chart of accounts, all income and expense accounts (those that start with 4 and 5) will now be set to zero.
I hope this helps!
