Sort the column with zipcodes and select only the cells that include zip codes with 9 digits without a dash between the zip and remaining 4 numbers. (Tip: do not select the ones that are 5 digit zipcodes)
On the Home tab, next to Number in the Number Format box , click the arrow, and then click More Number Formats.
In the Format Cells dialog box, under Category, click Special.
In the Type box, select Zip Code + 4
- Click OK
- Save excel and upload students