Any change to the form, or fields on the form are managed in Setup -> Object Manager.
What kind of things can be done?
From the Fields and Relationships section, you can:
- Locate the field, and change the picklist value order of a drop-down
- You can also add new drop-down values or remove any values
- Add net-new fields (watch this: “How to add or Modify fields on a record”)
From the Page Layouts section, you can:
- Re-order fields
- Hide fields
- Re-surface previously hidden fields