you change it in the uc_coupon module. Start with the module located at: http://drupal.org/project/uc_coupon

Then do the modification by searching for the function "uc_checkout_pane_coupon".

I have also attached my modificed uc_coupon module.

Note that I too cannot get the reports working but I am also looking into it...The coupon is working on Ubercart v0.8 w/Drupal 5.3 for me.

-Aaron