In short - by clicking 'Edit' in the desired campaign's row on the Campaigns page
You can easily edit any existing campaign with just a few simple steps:
1. Click on 'Campaigns' in the menu on the left side of the screen
2. Find the campaign you wish to modify, click
and then click on 'Edit'
3. The campaign editing page is the same as the campaign creation page, with the same steps.
Note that you do not need to complete the entire process to make a change. Simply click the next button, and the changes will be saved.
For guidance on a campaign's various options, please refer to this guide.