What happens when I change my iPhone 6 battery

As long as there’s no underlying issues with the device like it being disabled by entering passcode wrong too many times or something wrong with the board, your information will stay in tact. I’ve done hundreds of batteries on numerous models and the only times I’ve lost data were when the device was either already disabled or had a board level issue. Just make sure you use a quality battery from a good supplier. You can buy a kit from ifixi.com or sites such as Mobile Sentrix, Injured Gadgets, and Mobile Defenders. However those other sites don’t sell it as a kit with the tools they sell just the parts.