Navigate to the marketing tab on the left-hand side
Locate the campaign you'd like to see previous revisions for.
Click on the revision button on the right-hand side of your campaign, this is only available for campaigns within the draft stage as sent out campaigns can not be altered due to them being sent out to customers already.
A page will appear with a preview of your campaign on the right-hand side. Select a different revision on the left-hand side, this will update your campaign.
Once you have found the campaign you'd like to roll back to, click on 'Use Revision' on the top right.
You've now roll backed your campaign.