Go to your site's Network Admin page -- usually something like
example.com/wp-admin/network/. Go to Sites » All Sites.
Mouse over the site that you need to edit and select Edit in the menu that appears.
Now select the Settings tab. Scroll down to the bottom (or near the bottom, depending on whether you have plugins installed that add more settings at the bottom). You're looking for the Site Upload Space Quota setting. Put the appropriate number of MB in that box -- if you're updating a site's quota to 1GB, for instance, you'd put
1024 in the box.
If there's nothing in the box, it will use your network's default quota.