Navigate to one of the duplicate Account (or Contact) records that you would like to merge.
Locate
     the duplicate component (the box on the right in the image below) and
     click View Duplicates.

A
     window will pop up which allows you to view and select the duplicate
     records to merge. Check the box next to the records you would like to
     merge then click Next.

On
     the next screen, select the record you would like to keep as the master. If
     you are merging Contacts the master record determines which Household the
     Contact will be a part of. Any additional fields that are different
     between the duplicates will also be visible. Select the values that you
     would like to retain after merging. Click Next.

You
     will then be asked to confirm the merge. If you are sure you would like to
     continue, click Merge. 
The
     records have now been successfully merged.
Notes:
- You can
choose up to 3 records to merge at a time. If you need to merge more than 3
records, merge the first 3 and then repeat these steps until all are merged. 
- The
merged records not selected as the master are deleted. Ensure you have delete
access on the object of the records you are attempting to merge. 
 
