To merge two or more records, find them in any list view, select the duplicate records using the selection check boxes, and then in the blue bar that appears at the bottom of the page, choose 'Merge ...'
On the following page, you will have the opportunity to adjust fields that the records have in common with the correct values. Review these, then choose 'Merge!' at the bottom of the page.
If you receive the error message There was a problem merging the records then it's likely that you do not have the right permissions to edit each record. Make sure that you are listed on the team or are the assigned owner of all records you are attempting to merge in order to perform the merge. Or ask an Account Admin user to perform the merge for you.