How can I restart/delete analytics of a TL?

The only way to reset the analytics of a tracking link is to delete and re-create it.
Same approach if you want to reset the analytics of a project (after having removed every tracking link within it).

In case you want to delete the tracking link and the corresponding statistics after a specific period of time, you can leverage the "Timing" option for your tracking links, provided within JotUrl, and flag the "Disposal link" option within the latter.