How to we change one plan to another within a subscription

How to we change one plan to another within a subscription

Suppose i have 2 plans  - monthly 1250/pm  and annually 15000 per year.


I have subscribed to monthly plan and after 4 months i want to upgade to annual plan. 

or 

I have subscribed to annual plan and after  3 months i want to downgrade to monthly plan.

How this happen in subscription