How to replace old Ubuntu with new Ubuntu (Dual boot), not given option to "Replace"

I am dual booting Windows/Ubuntu. When I boot my new Ubuntu installation media I am not given the option to "Replace" the old version, jut either make a new partition or replace the whole disk. Is there a reason why I am not given the "Replace" old Ubuntu with the new version option?

(P.S. I currently have Kubuntu and it has stopped loading so want to replace it with a new version.)

Thank you