How to have my site use an updated css file (without query string)

I updated my bbPress css file in the theme folder
and I can see it is at http://www.christianboard.kr/wp-content/themes/dynamik-gen/css/bbpress.css.

But my site keeps using the old css file (apparently kept by some automated system) which is find at http://www.christianboard.kr/wp-content/themes/dynamik-gen/css/bbpress.css?ver=2.5.3-5249 (I don't know where this '?ver=2.5.3-5249' query string parameter comes from).

Is this a problem I can fix by myself, or do I need to contact my hosting service provider (e2) to fix it?

I tried deleting cache (using plugins like Hyper Cache, etc.), but it didn't fix the issue.

When I previously encountered a similar issue (I had changed the site directory but it was using the old directory) and contacted the hosting provider, they fixed it somehow. I am hoping there has to be a way to resolve this myself.

Interestingly, when I changed other files such as wp-config.php or another css file, there was no such referral to the older version of the file.