When updating a previous letter, it's important to review the criteria for the merge fields. This is especially true when using merge fields that input a total of all gifts or a list of gifts which meet certain requirements. For example, you may want to make the total of the gifts include the current year to date, rather than the date range used previously.
You can remove and then insert the merge field again to bring up the list of criteria to make those changes; however, there's an easier way to do this. Double click on the capital letter or symbol preceding the field name in brackets. This will open up the criteria window where you can make changes to the criteria and click okay to save it. Then, save your letter again.