After a while you will (or may not) hit the max limit of templates. This is where you find the delete button for all the different kind of templates you can create in Favro.
Remember! Deleted templates can not be restored.
Deleting a card template
Navigate to a board's menu, click More > Set card template... > Edit. Under the Edit option you can delete your card template.
Deleting a board template
When you create a new board you have the option to choose from already existing templates, including your own. To delete a template you have created you can click on the name of the template (in this case Development work board) > Sharing and more > Delete template.
Deleting a backlog template
To delete a backlog template, start by clicking on Create a new backlog. Then click on the name of the template > Sharing and more > Delete template.
Deleting a collection template
Click on the create collection-button, navigate to your template and click on the name of it, then Sharing and more > Delete template.
Simple as that! If you rather want to know how to create templates, please refer to this article.