How to delete lines from a table created by a custom module?
You will find an answer here.

Basically you can use Drupal's drupal_write_record() to let Drupal handle that.

Or you can run a SELECT query first and an UPDATE or INSERT based on SELECT's result.

EDIT:

Based on information gathered in comments, it seems that the "savingfactors" table has several columns and only one row. If the row is already present, with any value, then a simple UPDATE query is enough. Here's an example:

function savingsscore_admin_form_submit($form, &amp;$form_state) {
  db_query("UPDATE {savingfactors} SET saving_age21 = '%s'", $form_state['values']['saving_age21']);
  drupal_set_message(t('Your form has been saved.'));
}


That way you overwrite only saving_age21 field, ignoring others.

You can do the same with any other field you want to save.

There's an initial requirement: the row must exist. You can manually create it with a db client software, or you can implement Drupal's hook_install() inside your module .install file.