In general, it's cheaper to change the return portion of a RT ticket after all of the outbound segments have been flown although this might not apply to UA basic economy tickets. OP might want to do nothing before arrival in Europe and then ask again about the cost of changing the ticket. If OP decides to abandon the return portion of the ticket, it's normally better to cancel than to no show as no show often--depending on the fare rules and CoC--means that any remaining value of the ticket is forfeited.