There are a couple of ways to merge duplicates depending on how you're finding them.

In your contact list, expand Special Editing and click "Review Duplicates."

That will open a new window where Sumac displays all the contacts it thinks might be duplicates in your database. This window allows you to go through each potential duplicate and clean them up. You can expand each contact to see its potential duplicates, and if you do find duplicates, put an X next to each of them and click "Review and Merge"

Sumac will walk you through a new window where you can choose what data should be retained from each contact record. Simply click the value you want to keep and Sumac will put it into the new merged contact record. Once done, click "Merge" to combine the two duplicates into one!

Once done, you can continue working through the list of duplicates to merge them.

If you are working in your Contact list and see duplicates contacts, you can merge them on the spot by selecting both of them. Then, under Special Editing, click "Resolve Duplicates" to combine them.