How can I force browsers to check for a new version despite a caching directive max-age 30 days?

How can I force browsers to check for a new version despite a caching
directive max-age 30 days?

I had the following declaration in my website's root:
I think this tells browsers not to look for new versions of any file
unless it's older than 30 days - is that correct?
This was not my intention and I've since modified the instruction, but
while some browsers are now updating despite the old instruction, some are
not. Is there a way to force browsers to look for a new version with
immediate effect?
Thanks.