Switching from flea collar to topical?

How long do you need to wait when switching between treatments?

Cat has had a flea collar on for a while, but I don’t think it’s working that well. I want to switch him back to the monthly topical stuff. Can I just take off the collar and apply the liquid? How long to wait in between?

I’ve searched everywhere on this sub and cannot find a similar question. I don’t want to accidentally overdose him.