For Salesforce Data Entry Events there is no way of editing the entry source configuration currently.
The only way of "modifying" it is:
- Create a new journey version
- Delete the Entry Event
- Configure a new Entry Event
- Re-configure all activities using data of that event (decision splits for example)
A Salesforce Data Entry event is only editable as long as it isn't part of any journey or journey version:
NOTE Salesforce entry sources, including the Salesforce Data, Campaign, and Community entry sources, are only editable when no other journey or version uses them. You can't edit the object that a Salesforce entry source uses to decide who enters the journey. To choose a different object in a Salesforce entry source, delete and recreate the entry source.
(Source: Marketing Cloud Documentation - The Salesforce Data Event)