There isn't a specific Balance Sheet report available for the previous year, but there are two ways to view the information:
A Report:
Running the Balance Sheet as at Period report will show a comparison with Last Year to date, and so running it for the final period of this year will show last year's figures.
The Enquiry Screens:
Click Financials, Enquiries, Balance Sheet.
Change the search criteria to show period 12/13 for the previous year. This will then show the previous years' Balance Sheet.