Unfortunately, there isn't an 'Undo Upload' button.
To delete test sheets from an assessment, go to the 'Responses' tab on the 'Results Analysis' page and select the sheets you wish to delete.
Hint: The default view will have the results listed in "Upload Order". You can quickly select test sheets, by holding the 'Shift' key and pressing the 'Down Arrow' or 'Up Arrow' key.
Once selected, click the 'Delete' button then 'Save Changes'.
You can then re-upload the test sheet to the correct assessment.