use commad:
// For version 15
bench get-app --branch version-15 erpnext
// OR
bench get-app --branch version-15 https://github.com/frappe/erpnext.git
// For version 14
bench get-app --branch version-14 erpnext
// OR
bench get-app --branch version-14 https://github.com/frappe/erpnext.git
Reference:
Thank You!