Let's go through the process step-by-step, to start you can watch the following video. It guides you through the steps you need to take to create an email campaign and the steps to edit that are very similar:
To change the body of your emails first click on the email campaign title from the list on your Campaigns tab to access the editor:
You will be taken to the Build Campaign step (Step #3), where you can make changes to your campaign templates and email settings. If you click the orange Email circle, the system will let you adjust your content and settings:
The editor will appear as soon as you click on it, allowing you to make any edits! Once you have made all the changes you need, click on the orange Save button.
If you don't want to change the template, but wish to add a new product instead, please click on Step #2: Select Product.
When that page opens, you will see all active products along with the ones that have not yet been added to another campaign already. If the box is ticked, then means it's added to the campaign...so make sure the box is ticked for a product you want to include in the campaign: