I am using iphone 6 and my battery Max capacity is 1118 mAH and design capacity is 1751? I am using iphone 6 ios 9.3.3 my battery Max capacity is 1118 mAH and design capacity is 1751 ?

Why its Max capacity is is not 1751 what to do to fix it.
Your battery is worn out 

Batteries, particularly lithium-ion batteries loose the ability to hold charge as they age. The biggest indicator of how old a battery is is not just the age but the charge cycle count, which is the number of time the battery is charged to 100% and discharged again back to 0. Note that this doesn't have to happen all in one charge but is cumulative. 

Apple has some good information on it here.

645 charge cycles is a lot. The current estimate of the lifespan of an iPhone battery is  300 - 400 charge cycles There is nothing you can really do to fix this except use the iPhone in such a way to maximise the remaining life of the battery. More information can be found here.