How do I switch an active cell phone I have to another cell phone?

I currently have a phone with Verizon that in its last days.I never ordered a replacement phone because I have a phone that's with the same company and I feel that I can reuse the old phone. So my question is how do I switch service from the broken phone to the reusable cell phone?

Go online & sign in.

On the right side click on activate a phone.

If you call them or take the phone in they will charger you 15 dollars, if you do it yourself its free.