My iPhone 6 g restarts every now and then

What makes you think so?

Even the iPhone 6 is a pretty old device, the failing battery might be the problem. Especially, if you haven't replaced it. Batteries chemically age because of using them and when they degrade a lot, they don't just lose the original capacity but they also can't provide your iPhone with enough power to perform tasks correctly.

What is your Battery Health under Settings > Battery?

Hello.

How old is your iPhone 6?

Have you ever replaced the battery in it? If you did, was the replacement performed at the Apple Authorized Service Provider or 3rd parties?

Schedule an appointment at a Genius Bar or the nearest Apple Authorized Service Provider to replace your battery.