you can use this:

(0, 0,  { values })    link to a new record that needs to be created with the given values dictionary
(1, ID, { values })    update the linked record with id = ID (write *values* on it)
(2, ID)                remove and delete the linked record with id = ID (calls unlink on ID, that will delete the object completely, and the link to it as well)
(3, ID)                cut the link to the linked record with id = ID (delete the relationship between the two objects but does not delete the target object itself)
(4, ID)                link to existing record with id = ID (adds a relationship)
(5)                    unlink all (like using (3,ID) for all linked records)
(6, 0, [IDs])          replace the list of linked IDs (like using (5) then (4,ID) for each ID in the list of IDs)
Example:
   [(6, 0, [8, 5, 6, 4])] sets the many2many to ids [8, 5, 6, 4]

This will help you to update your many2may and one2many records as well.
If you want to change a particular field in a one2many record, then do a filter on the one2many  field. Here is an exemple:

@api.onchange('x')
def onchange_field_in_onetomany(self):
    b = self.onetomanyfield.filtered(lambda a:a.afield==condition)[0]  # [0] because the returned value is a recordset
    b.x = newupdate_x
    b.y= newupdate_y