Testing postGIS update in a green instance

I just jumped into the deep end on this, so please bear with me.

I'm trying to update the PostgrSQL engine on a db as safely as possible, which AWS says means I should use a blue/green deployment.
I couldn't update while creating the deployment, so I created one with the same version as my current db. I couldn't update this one either, and it appears to be due to postGIS needing an update first (3.3.3 to 3.4.0). I want to try the update on the green instance in case it breaks anything, but I can't do it since it is a read only replica.

Is my only option to update the blue instance and hope it doesn't break anything? That seems wrong, so I suspect I'm missing something.

Thanks