After adding custom sample fields it is possible to edit these fields. Please note that editing a custom field a new custom field will be added. In the user interface it will look the same and interact as the same field. When exporting samples or importing samples using the Data Migration Import tool it these fields will be visible. 

To update a sample field go to Sample Types in Configuration and click the Edit icon.

Click on the edit icon from the field that you want to edit.

A pop-up will open, you can make and save your changes.