Contacts merge from Campaigns to CRM

Contacts merge from Campaigns to CRM

I was previously using a different emailing software and imported all of those contacts into Campaigns. I am now wondering how to merge the contacts from campaigns back to the contacts in CRM? Many of the contacts in Campaigns are already contacts in CRM so I want their information to merge together. The contacts that are not in CRM I do NOT want to add as contacts in CRM, only the existing contacts merged.  

Please help! 

-Allison Rowe