Can I increase post_max_size?

I see the post_max_size on my server is 8M and I’d like to bump that up. Is there any way to do this from my end with an htaccess file or something?

I’m not necessarily sure that post_max_size would be all that’s involved either. Would I need to also alter the upload_max_filesize and memory_limit as well? All moot if it’s not possible from my position…

If this_is_not possible, how would you suggest I handle an upload form so I (user/client actually) can upload up to 20MB through it?