First read all these
-
for custom app
Migrating to Version 14 · frappe/frappe Wiki · GitHub
Migrating to version 15 · frappe/frappe Wiki · GitHub -
for erpnext
Migration Guide to ERPNext version 14 · frappe/erpnext Wiki · GitHub
Migration Guide to ERPNext version 15 · frappe/erpnext Wiki · GitHub -
other resources
https://www.youtube.com/watch?v=sz2xBW9l_cU
https://www.youtube.com/watch?v=9S3APcmAql8