When a project is in Undeployed

and no sessions (responses) have been collected you can delete it but doing the following:
Delete data from projects with collected session
When a project is deployed

with collected session you will need to first close the project.
1a. This can be done from the main menu via the quick action button "Close project".
1b. Another way of doing so is to go in the project go to Current Status>Click on dropdown bar>Close Project.
Project will then turn to the closed status

, then you need to go into the project and remove the collected sessions.
2. Within the desired project go to Data>Delete all.
The removal of sessions is permanent, only remove sessions from projects where the sessions are not needed anymore.