It is not possible to change the donation campaign associated with a donation form. However, you can easily recreate the donation form under the correct campaign by copying the donation form and assigning the copy to the correct campaign.  
  1. Fundraising>Donation Management
  2. Click All Donation Forms
  3. Click Copy next to the appropriate donation form
  4. Enter Form Name field and select the correct Donation Campaign
  5. Click Finish
Please note that the merchant account where funds are sent is defined at the campaign level, so there would be no way to move any transactions made under the incorrect campaign to a different merchant account if needed.