Do I need to cancel a flight or just not turn up?

As long as you do not intend to try to use the 2nd part of the ticket (the return portion), you’re fine with just not showing up. Nothing will happen and you will not be charged. I’ve seen many cases where passengers buy a cheap roundtrip ticket, when they only want to use one-way.

https://www.tripadvisor.co.uk/ShowTopic-g1-i10702-k11208484-Cancel_or_don_t_show_up_for_flight-Air_Travel.html#:~:text=As%20long%20as%20you%20do,want%20to%20use%20one%2Dway.