There is not a script for upper case in HTML field scripting. However, there are two other options for accomplishing this:
1) Use the text reformat option found on the Format tab of the field. You can set this to "UPPER." The text reformat will be honored for that specific field even when the field is used in a field script.
2) Use VB script. The following script will set the contents of the field to use upper case letters. (Note - you would replace FieldName with the actual field name.)