In Colorado anyone can officiate at a marriage - even the bride and groom. All you need to do is to get a marriage license and have someone sign it (or sign it yourself) as the officiant. You can have a lawyer do it, or you can have anyone else that you wish. Alternatively, you can live together and agree to be married and you will be legally married in Colorado. Having the marriage license will make it easier to handle the name change.
You can reach Harkess & Salter LLC at (303) 531-5380 or info@Harkess-Salter.com. Stephen Harkess is an attorney licensed in the state and federal courts of Colorado. This answer is for general information only and does not create an attorney client relationship between Stephen Harkess or Harkess & Salter LLC and any person. You should schedule a consultation with an attorney to discuss the specifics of your legal issues.