You can only change your contract by contacting Telone and ask them to move you to another package of yourchoice. Once your data runs out, l think your options will be to buy top up bundles.
Im not sure how flexible the telone systems are, but if you buy a bundle higher than what you signed for, it either credit your account then put the extra bit left for your following month purchase, ie, credit your account or it rejects it totally if they use serial numbers to match packages.