How to edit manual rates programmatically

If you create a manual exchange rate, let's say, with machine name example,

you then get the configuration file which is exported and named
commerce_exchanger.latest_exchange_rates.example.yml

Let's say you manually defined the relationship on creation between HRK and EUR currency.
The above YML configuration file then have contents as shown here:

rates:
  EUR:
    HRK:
      value: 7.5345
      sync: 1
  HRK:
    EUR:
      value: 0.1327
      sync: 1

Programmatically you can modify these by using the config service.

Alter ratio EUR / HRK from 7.5345 to 1000 would look something like this:

  $rates = \Drupal::configFactory()->getEditable('commerce_exchanger.latest_exchange_rates.example');
  $rates->set('EUR', ['HRK' => ['value' => 1000, 'sync' => 1]]);
  $rates->save();

But as well you can use UI in Drupal to alter values.

If you have an external rate source, I would suggest creating a plugin.
Usually, this boils down to a few lines of code (see examples in the main module or readme)

#2 is no longer applicable, from version 2 we don't use any more Drupal configuration to store exchange rates.

Now it's dedicated database table `commerce_exchanger_latest_rates`

See 3366705 🐛 Latest exchange rates shouldn't be saved in config Fixed

You can use https://git.drupalcode.org/project/commerce_exchanger/-/blob/2.0.x/src/E... for storing and retrieving data.