Can i upgrade to higher subscription plan?

Login to your account, go to general setting and click >>>> Subscription manager  .. 

Click on Topup to renew your existing plan, or select Upgrade to switch to a new or better plan.

FInaly Enter number of month, click pay Now .. your wallet will be debited and your account subscribed.

Can i upgrade to higher subscription plan?  Yes you can ..

What will heppen to my already subscribe plan ?,,, it still runing ..?

if you want to upgrade the system will cancel your current plan and retun the money back to your wallet then deduct full pyment for the new plan.

if $15 is for 1month (30day) sub .. it will deduct the days you've used and refund balance

As it upgrades you to a higher plan, it deducts the remaining sum from your wallet balance and makes up the difference by charging your wallet a lower cost.