Top-up purchases are final. This page explains exactly what that means, because the rule is stricter than for the rest of what Flex Mobile sells.
The rule
All top-up purchases are final and non refundable. They can't be transferred, exchanged, resold, or redeemed for cash, merchandise or services. That applies even if you never use the data.
Top-up data also resets on the first of the calendar month rather than lasting until your plan renews, so buying a large top-up in the last week of a month is rarely good value.
How this differs from the rest
Prepaid plans can be refunded for unused full months. Top-ups have no equivalent window.
Why it's worth checking before you buy
Look at your data usage and how many days are left in your cycle first. If your cycle resets in a day or two, your full speed returns anyway and the top-up buys you very little.
If you bought one by mistake
Contact Flex Mobile support and explain what happened. The policy is firm, but it's better to ask than to assume, and support can at least check what was applied to your line.
A better long term fix
If you're buying top-ups regularly, changing plan is almost certainly cheaper than repeating the purchase every month.