IPhone 6s battery drain and jump

I bought a second hand iPhone 6s yesterday and set it up as a new iPhone, updated to most recent IOS and noticed the battery began to drain almost second by second. Checked battery health, says 87%. Did all the tips I could find on internet, stopped it draining second by second, but now it will jump percentages. It had a full charge and I started using the phone one hour ago, battery is now at 38% after jumping down from 62%.

I doubt I will get the money I paid back so is there anything I can do to sort the battery? Does it sound like it needs replaced?