Reset Macbook Air 2019

I want to factory reset my Macbook Air 2019 with MACOS Catalina v. 10.15.7 and I see a Macintosh HD and a Macintosh HD - Data. Do I need to erase both or just the Macintosh HD? I noticed that the "data disk" has much less storage space than the "HD original". I am trying to update my Macbook Air and it says it doesn't have enough storage space for the update. Thank you in advance for replying.