Can we upgrade/update Frappe HRM: hrms 15.43.1 → 16.0 Beta only, within ERPNext V15? My ERPNext V15 15.59.0 & Frappe V15.66.0
No it v16 has many breaking changes as well as changes in UI, it will not cope up with v15, not recommended to upgrade only one app, for v16 beta, your frappe should be updated to v16 beta.