Anyone ever try a credit repair expert?

I’m looking to pump up my credit score from 640-to 700 to get a better interest rate on my mortgage. I’ve heard opening new credit card accounts can help (some say it could hurt) the score but i doubt that would be enough. I surfed the internet and read about credit repair experts that could help. Has anyone tried these guys? Do they really help and if they do how much can they help. The agency i talked to there name was Lexington, but not sure if they can help as much as they say they can.
Thanks for the advice.