Assuming the user rest resource is enabled and PATCH
requests are configured, also that the requesting user is authenticated to update the user, then:
send a PATCH
request to: https://example.com/user/$uid?_format=json
Where $uid
is the user id to update (e.g 12)
The request body should look something like this (json format):
{ "field_motherid":
[
{"value": "new_value"}
]
}