You can completely delete a project, client, or task from the Add Timer screen (get there by tapping "Add timer" from the main screen). Tap in the "Project," "Client," or "Task" entry field, and a list of existing items will appear. Swipe right to left on the item you want to delete, then tap Delete.

Deleting a project, client, or task can be handy if you made it accidentally or made it with a misspelling and have never used it. But be aware that if you delete a PROJECT or CLIENT that was used in a timer to create time entries, THAT TIMER AND ALL TIME ENTRIES CREATED WITH THAT TIMER WILL BE DELETED and cannot be recovered. Deleting a TASK will remove that task from all time entries it was associated with, but the time entries themselves will not be deleted.

If you just want to hide a timer from your main screen, you can archive it. See the FAQ entitled "Archive and restore timers." You also have the option of editing a project.  See the FAQ "How to edit a project name on the app."