You can modify (or edit) fields by navigating to the Layout tab and clicking on the settings cog on the right of the field. You will be able to change the Field Label (which is an identifier so you can find it in the Details Tab later) and also add a “Custom Field Tag”. You will not be able to change the field type as this can only be assigned when you create the field.
Custom field tags are the prefixes such as P: or E: that you commonly see in email signatures. If you would like to have these prefixes in your signature, make sure you tick the “Custom Field Tag” box for every field you wish to have the field tag.