How can I do it
what will happen if I upgrade my phone
iPhone 6, iOS 12
An iPhone 5S 6/6plus can't be updated beyond 12.5.6, it doesn't have the hardware needed to run anything higher.
If you need a higher iOS version, you will have to purchase a newer phone.
If you are at a lower version than 12.5.6, upgrading to 12.5.6 will update you to the latest security patches.