Before you perform the merge, if you have any required and dependent fields that are empty you will need to go into studio and set these to have the Duplicate Merge value of Disabled
Select the records you wish to merge using the checkboxes to the left in the relevant list view
From the dropdown above the checkboxes select Merge
On the resulting screen select the primary record by dragging the bar above the records
All related records will add to the primary record and none of these will be deleted
Select the information from the fields of each record you wish to keep using the radio buttons
Click Save
Note: To be able to merge records, you will need to have permissions configured in the system that allow you to delete, as a merge deletes the secondary record.