Where is the search and replace feature?
Not sure about "Search and Replace", but you can do the above from the "Update multiple contacts" task on the contacts returned on Advanced Search.


All you need to do is add your field(I think it's IM Screen name in your case) to a profile (Created from Administer -> Customize Data and Screens -> Profiles).
Use Search -> Advanced Search to filter and get your contacts -> Select Update multiple contacts on the Action dropdown.
Select your profile where you've added the field in step 1.
Enter the new value for the contact(You can also make use of Copy icon to copy text to all the rows) and click Update Contacts button.