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 
   
  

From the Page Layouts  section, you can:

• Re-order fields
• Hide fields
• Re-surface previously hidden fields