get list of all Batches. for every batch do your changes .
batches = frappe.db.get_list('Batch')
for b in batches :
doc = frappe.get_doc('Batch', b["name"])
doc.expires_in_days = (your code here)
doc.save()
you can open (bench console) and write the code there to test it . if it works move it to a scheduler event.
some erpnext python documentation :
Document API
Database API