@seema94, When you encounter electronic devices, that has an internal battery, and the device promptly power off when unplugged from charger, you have a device where the internal battery is toast and needs replacing.
Lithium batteries (as all batteries) have an expected lifetime, and that is given in years or charge cycles (whichever comes first). Replacing internal batteries can be anything from incredibly simple, to very, very tough!
Replacement battery for the device you’ve chosen aren’t carried by iFixit, but they’re plentiful online, using a simple Google search. When buying replacement parts, be sure that you do some research and don’t just select the first and / or cheapest! Most times it’s far, far better to pay a few extra bucks and purchase from a reputable vendor. There are so many sellers that sells replacement batteries, that are in fact pure scams!