Where to change the sender email address on a Registration form depends on whether the organization is using Simple Confirmation or Advanced Confirmation. 

If using Simple Confirmation:
  1. In Sphere, go to Communities > Forms: Registration
  2. Hover over the form and choose Website Info
  3. Click Registration Form Options and click next until step 5 of 5
  4. Change the Sender Email Address and click Finish
  5. Navigate back to the Webinfo Checklist and click Publish This Event Now
If using Advanced Confirmation:
  1. In Sphere, go to Communities > Forms: Registration
  2. Hover over the form and choose Website Info
  3. Click Registration Form Options and go to Step 5 of 5
  4. Select the Advanced Configuration link
  5. Select Edit email
  6. Modify the From Email Address displayed at the top of the form
  7. Click Save
  8. Navigate back to the Webinfo Checklist and click Publish This Event Now