I have a holiday booked with TUI for May which has now been cancelled. Before being cancelled there was an option to amend the booking to any future booking free of charge. Think with TUI when I last checked it was if your booking was up to 30th June (a date which I assume will extend as restrictions continue).
Would assume Jet2 have similar options. (and TUI are selling holidays through Summer 2021 so the option to rebook for next year, not just later this summer)