If you have a model that belongs to another model, for example, account belongs to user. This would be defined in the account in the following way.
public function user() {
return $this->belongsTo(User::class);
}
If you want to transfer the account to a different user, you could do the following:
$account->user()->associate($newUser);
$account->save();
In the database, this has the effect of changing the value of the user_id foreign key found in the account record.