You don’t need to say anything to Republic.
When porting out of Republic you can find your account number by going to https://account.republicwireless.com/account/phones and then under the phone you want to move select “I Want To” and then “Transfer My Number Out” to see your account number. You’ll also need to create or set your PIN following the directions here: How to Transfer Your Number from Republic Wireless – Republic Help You’ll have to ask your new carrier for their process to port your number
You do not receive a credit back for unused time. When you transfer out any remaining time is lost.
You don’t have to do anything to cancel, when your number ports out your line automatically cancels.
Out of curiosity, what’s causing you to leave Republic?