You can upgrade your plan in the "Upgrade Plan" section of your Company Settings, or by clicking Upgrade Plan in the company drop-down from the upper-left corner of your dashboard.
You have the choice of paying for additional upload quota per GB or upgrading to a higher plan (if one exists).
Note: You must have Admin privileges for the company you are attempting to upgrade. Users with "Contributor" or "Manager" privileges will not see the option to upgrade.