Grades can be posted from Administration, Post from FAWeb. On the General tab there is an option to "Update existing entries".  When this option is marked, the newly posted grades will overwrite existing grades. If this option is not checked then the original values will not be overwritten. If grades need to be updated for some students in a class, but not others, the grades must be edited manually in the grades Module.

NOTE: Existing grade values cannot be overwritten with a blank value using this option. Posted grades can only be changed to a blank by manually editing the value in the grades module.

For more information, refer to How to post from Faculty Access for the Web.