need to back up old iphone 6 and than remove device?

Backup:--> How to back up your iPhone, iPad, and iPod touch – Apple Support

Remove device? are you going to sell it, or give it away, or have it repaired? Should be this case, after backing it up, use these instructions:--> What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support

Regards

Giulio

Okay, see section "How to back up with your computer" in this support guide:--> How to back up your iPhone, iPad, and iPod touch - Apple Support

Regards

Giulio