Did you get a promotional text saying you could do this? If so just respond to the text accordingly.
If you didnt receive a promotional text then you can change your plan to one with more data or if you are on a shock free data plan, you can get a data top up.
I already have a plan for 500 MB data and I could use more. I received a promotional offer to increase my data by 200 MB for $5.00 or an additional 500MB for $10.00. Is this not correct? I believe the addition 200 MB would be sufficient.