Since you weren’t the previous user of the phone a factory reset to start fresh is best practice: Factory Reset. Post reset, it’s pretty much a matter of following on screen prompts. Some detail follows:
Republic offers a great walkthrough here: Activate | Republic Wireless. You will be prompted to sign into the appropriate Republic account. To upgrade an existing line to a new phone, select “replace existing line”. You should see the phone number you want to replace listed at this step. Continue through the activation process, then you’re all set! Detailed activation guidance here: Activate My Phone.