Moment.js is very easy to customize. In general, you should create a locale setting with your customizations.

moment.locale('en-my-settings', {
    // customizations.

You can remove a previously defined locale by passing null as the second argument. The deleted locale will no longer be available for use.

moment.locale('fr'); // 'fr'
moment.locale('en'); // 'en'
moment.locale('fr', null);
moment.locale('fr'); // 'en'

As of 2.12.0 it is possible to create a locale that inherits from a parent locale.

moment.defineLocale('en-foo', {
  parentLocale: 'en',
  /* */

Properties that are not specified in the locale will be inherited from the parent locale.

As of 2.16.0 it is possible to define a locale with a parent that hasn't itself been defined or loaded.

moment.defineLocale('fakeLocale', {parentLocale:'xyz'})

As of 2.21.0 when attempting to create a moment with the newly defined locale, moment will attempt to lazy load the parent if it exists. Failing that it will default the parent to the global locale.

As of 2.12.0 it is also possible to update a locale's properties.

moment.updateLocale('en', {

Any properties specified will be updated, while others will remain the same. This function does not affect moments that already exist. Note that calling updateLocale also changes the current global locale, to the locale that is updated; see this GitHub issue for more information.

To revert an update use:

moment.updateLocale('en', null);

2.12.0 deprecated using moment.locale() to change an existing locale. Use moment.updateLocale() instead.