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.