How do I roll over my unused data?

I understand your concern. However, regardless of how cheap an MTN monthly data plan is, you will be able to roll over unused monthly data bundle with another monthly data plan with a different price range.
One sweet thing about the MTN data rollover policy is that it allows you to roll over unused data bundles within the same threshold or data bundle type; i.e. rollover unused data bundle of
1. Yealy to Yealy
2. Bi-annual to Bi-annual
3. Quarterly - Quarterly
4. Bi monthly - Bi monthly
5. Monthly to Monthly
6. Weekly to Weekly
7. Daily to Daily