Yes, you can upgrade. Upon upgrade, your bill will be prorated based on your usage of your previous plan and the new plan.
Digi Fibre Support
Support Detail
Can I upgrade to a higher value Digi Fibre plan within the contract period?
How do I upgrade my existing Digi Fibre plan?
You may walk-in to the nearest Digi Store to upgrade your Digi Fibre plan.
Can I downgrade my existing Digi Fibre plan within the contract period?
No, downgrade is not allowed within the contract period.