Changing Duration of Only One Note in a Bar

I want to change the duration of one note in a bar to suggest that you would hold it down whilst the other notes in the bar are played by the same hand but it won't let me extend the duration of one note without changing the duration of the note it's attached to.

I want to make this G a semibreve while the rest stay crotchets but it won't let me change the duration without changing the C as well. Is there a way around this?