f you're using XSSF workbooks, you can re-evaluate all formula cells as follows:
XSSFFormulaEvaluator.evaluateAllFormulaCells(workbook)
A similar API exists if you're using an HSSF workbook:
HSSFFormulaEvaluator.evaluateAllFormulaCells(workbook)
Or, depending on exactly how you're set up and what specifically you're trying to get accomplished, you should be able to find your solution here