You can do it a couple ways. Back up using your carriers app and you should be able to restore them when setting up your new phone if you want to use the carrier to manage contacts. Or save them as a csv on your PC, create an Outlook account on the web (sign up using the same Microsoft account you'll use with your phone), and in the People section go to Import Contacts and upload the csv. You may need to add Outlook in the "filter contact list" setting of the People app on the phone if you set up your carrier's contact app as your default.