You should get an email of your options. However if you don't do anything you could be lumped onto the full retail price of broadband.
It absolutely will NOT just end, if you want to cancel then you need to call the broadband team. If you want to stay and get a cheaper discount then you need to call the broadband team.
If you didn't get the 12 month email reminder of your options and there is nothing in your spam email folder, then see if it's under Broadband & Calls > Messages from your online NOW account.
Should you find the email in question read it very carefully, especially the bit about "do nothing" where a lot of people seem to miss the bit at the very bottom of the email option where it says £X amount discount up until X date.
If your discount runs out in 2023 and not 2024 in twelve months time, then that discount rate gets added to your bill each month after the expiry date of the discount date finishes if you do nothing.