Lakshmi
There is no direct way of updating the internal data with a spreadsheet. Its not even recommended to use a spreadsheet as a datasource because any change in the file will mess up all the logics.
Anyway, if excel is only form how you can get data then you can think of 3rd party options like InfoBurst where you can cache the data of an excel file in the server and the Xcelsius can access the cached data using custom connector (provided with the product).
http://www.infosol.com/xml-data-cache-xdc/
If you have BO - you can create WebI using excel and then use BIWS to get data to Dashboards.
Runali