You shouldn't need to do anything as long as you turn 70 before the end of the year. The extension applies to all licences and entitlements (the entitlement to drive is what you're applying to renew when you turn 70), except provisionals. The DVLA extension applies to licences and entitlements which expire before 31 December 2020. So, for example, if your licence expires on 21 December 2020, it won't need renewing until 21 November 2021.