In Organization Settings, select Projects in the left-hand navigation. Then, next to the project you want to delete, click the three dots and you'll get the option to transfer ownership or delete the project:
If this process doesn't work...
One thing to note is that you cannot delete a project while it has any child objects. This includes any submission tokens, integrations, or attributes. If you're running into errors when attempting to delete a project, go into the Project Settings for that project and remove any child objects in any of the sections in the left-hand navigation.
The most common lingering item that prevents project deletion is a leftover submission token.