You could do that, but upgrading from 13 to 15 is a big change. It will take you a while to figure out what all has changed and where Apple moved stuff around to. The only factor that might prevent the upgrade is how much free space is on your iPhone 6s. So prior to attempting the upgrade check to see how much free space is required. If you don't have enough free space you might be able to delete some things to free up more, but the closer you are to running out of storage, the less sense it makes to update.