Instead of manually typing pieces of a cell reference, use this quick mouse trick to extend the reference when modifying a formula.
You probably know how to edit a formula - press [F2] and make changes. If you're like most users, you press [F2] or click inside the formula bar and then use the keyboard to change cell references. For example, to include values for Canada in the sheet below, you'd press [F2] or click the formula bar and then highlight the 6 in the B6 reference and type 7.
When editing a formula, you might have noticed that Excel outlines the referenced range. You can extend that range to update the formula's reference. Hover the mouse over one of the corner handles until Excel displays the double-arrow pointer. Click the handle and extend it to include the appropriate cell(s). Doing so automatically adjusts the respective reference in your formula. Press Enter to enter the modified formula.
You can move the reference instead of extending it by hovering over a border instead of a corner handle. Excel will display the four-arrow pointer, which you can then use to move the border.
Extending the range is certainly easier than trying to highlight characters one at a time.