The auto-complete list for Outlook 2010 and above is stored in .dat files in the Roam Cache folder under the Local App Data folders. You can access this folder by typing or pasting the following command into the address bar of Windows Explorer and pressing enter.
You can use a utility to extract the addresses from the auto-complete stream and insert them into the new auto-complete stream. Or you could rename the .dat file with the name of the new .dat file and replace the old with the new .dat file.
Step 1 : Copy the old .dat file in desktop and rename the new .dat file same as the old .dat file.
Step 2 :Then Copy old.dat file and Paste it in the location of the new .dat file.
Step 3 : Then it will ask for the Replace the file or remain both files because both the file is as same name, you have to choose Replace the file.
Step 4 : It will Replace the new .dat file to old .dat file and all the contact data will get updated in the outlook.