To redeem the offer for a free year of Apple TV+, you would have to have bought a qualifying new Apple device from Apple or from an Apple Authorized Reseller, and have activated that device (first use) between 3 months ago and July 1st. For devices activated on or after July 1st, the offer would be for 3 months free.
This offer can be redeemed once per user or per family (in case of Family Sharing).
The offer should be redeemed on your new qualifying Apple device.
For Macs, remember to additionally sign in to the Apple TV app (menu Account), for the redeem button to show.
Read here how to redeem: Apple TV+ offer - Apple
Read here about the conditions: Apple TV+ offer - Terms and conditions - Apple
Read here if you have trouble redeeming: If you can't redeem your promotional trial of Apple TV+ - Apple Support