Can I edit the fields of a Target Data Extension in DESelect Segment?

yes, you can this could be useful if you have created a target data extension before (from within deselect or sfmc) but realise you forgot to add a field deselect makes it easy to just add a field to your target data extension without leaving deselect on the output screen, perform the following steps click on the tools button on the right and choose edit data extension on the screen that opens now, you can click the pencil icon on a field to change the properties of that field you can change settings like name, length, default value, and indicate if a field is nullable / primary key add a new field to the target data extension by drag dropping a field from the available fields section to the section on the right double click a field in the available fields section use the add all fields button to add all fields of a selected data extension click on the update target data extension button to save the changes to the data extension, or click cancel to return without changes