In Drupal 8 / Commerce 2, you can add a string override to your settings.php file, as such:
/**
* String overrides:
*
* To override specific strings on your site with or without enabling the Locale
* module, add an entry to this list. This functionality allows you to change
* a small number of your site's default English language interface strings.
*
* Remove the leading hash signs to enable.
*
* The "en" part of the variable name, is dynamic and can be any langcode of
* any added language. (eg locale_custom_strings_de for german).
*/
# $settings['locale_custom_strings_en'][''] = [
# 'forum' => 'Discussion board',
# '@count min' => '@count minutes',
# ];
$settings['locale_custom_strings_en'][''] = [
'Checkout' => 'CHECKOUT or GET QUOTE',
];
Specifically, this part:
$settings['locale_custom_strings_en'][''] = [
'Checkout' => 'CHECKOUT or GET QUOTE',
];