How do I convert my data to cash, I mistakenly bought subscribe to the line I don't use for browsing

You can not convert data to cash unless you've found a customer willing to give you cash for your data instead. However, if you can visit any MTN centre close to you, you will be advised on what can be done if your phone hasn't used the data for auto-updates. On the other hand, the data you mistakingly bought can be transferred to your preferred browsing line dialling *312# and selecting option 7, which is "gift data."