There are two ways in which you can re-use the template used in a previous campaign.
Copy Campaign feature
Go to CAMPAIGNS screen.
Click on the 3 dots on the right hand side of the campaign whose template you wish to copy.
Select the Copy Campaign option from the pop-up.
Fill in new campaign details such as New Campaign Name, New Campaign Description etc. You will also have an option to attach the contacts associated with the campaign you are copying by ticking Copy Campaign Contacts into New Campaign.
Go to the newly created campaign's Campaign Details page and update the settings as required. You can of course edit the content in the new draft campaign as well.
Bring in the contents of another email message
Create a new Campaign as per usual.
Go to Email Content & Test page. Select the appropriate campaign name from the list provided by the Bring in the contents of another email message option (see image below). The list lets you choose all campaigns that you have previously sent, including those that were archived.
3. Click on the GO button once you have selected the desired email. The template will now appear within the Email Editor, and can be edited as required.