Drupal 8 alter existing field setting
To alter how to insert content into the field, define your own field widget and then apply the new widget for the existing field in "manage form display" of the content type.

Field Widget

To alter how the field is displayed, define you own field formatter and apply it in "manage display" in the same way:

Field Formatter

When you start with a field widget or field formatter, the fastest way is to copy an existing one out of core and modify it or use drupal console to generate the scaffolding.

If you only want to modifiy an existing widget/formatter, then extend it, not write a complete new one. This is a good explanation, how to do this:

http://www.sitepoint.com/creating-custom-field-formatters-drupal-8/