If you have experience then you can, if not then you risk damaging not only your iPhone battery but possibly killing your iPhone to, there are various guides on the internet but some are not as good as others, so we would also recommend you watch a few. 

The biggest problem with iPhone battery replacements, when you opt for the DIY option, is the quality of the parts, the mistake most people make is they go for the cheapest iPhone battery they can find, there is a reason why its cheap, its using low grade components, it wont last very long and its likely dangerous, lithium isn’t a cheap battery to produce.

So you may think you will buy an original apple iPhone battery, these are mostly fake, apple do not make parts easily available. Apple have recently started a self service repair store for original parts, but this is only available for the most recent models, and is often as expensive as paying apple to do the repair

We use only genuine iPhone batteries when available or high quality manufactured iPhone battery replacements, which pass European safety measures that will act and last just as long as the original. To often we have seen competitors using cheap eBay batteries, just to make an extra few pound at your expense.