If you are running out of data before reset day (before the next cycle begins), you can choose to Add Extra Data on Current Plan in User Center.
Please be aware that the extra data you purchased will be set to zero at the beginning of the next cycle, or at the end of your subscription period.
Alternatively, if you continuously find your current data cap is not enough for your monthly consumption, you can upgrade to a higher level plan, which means higher data cap per month.