How can we edit default fields through custom app so that whenever that app is installed in a new site, those changes are reflected there.
You might want to write a patch
https://frappe.github.io/frappe/user/en/guides/deployment/migrations
https://discuss.frappe.io/t/use-property-setter-to-override-doctype-name/4990/2
Property Setter or Patches what is better for the above?
Depending on your requirement, Patch will be executed only once, Property Setter, on the other hand, will sync after every migrate.