For Non root users
Start by turning on your mobile data first (important), then go to Settings > Data usage > tap on the present date cycle > Change cycle > Select today’s date. This will reset your data from the present date. You can still see the older data usage, though.
For Root Users
Start by opening your file explorer (if you don’t have any, install ES file explorer) and then go to /data/system/netstats. Under the netstats folder, delete all files and reboot your device. This will clear everything under ‘data usage’.