iPhone battery health is very low

The link on top of this page which you had visited, before posting this question here, is shown under the "Related Article” that may have given you an answer.

You need to mention which part of the article was not clear to you.

With the limited information that you have provided, It is hard to even guess the problems faced by you. 

You may get faster and better responses to your query by --> Writing an effective Apple Support Commun… - Apple ...

Please re-post with detail so that someone else can help.

If your Battery Health / Maximum Capacity is 80% or less, you need replace the battery on your phone at the Apple Store or an authorized Apple Service shop. About $69 U.S.

Find Locations - Apple