Once created, you can edit a Global field anytime. However, removing a field, changing its data type, or enabling the Multiple attribute may result in data loss.
Additional Resource: If a Global field is used in published entries within a content type, modifying its schema may cause data loss in those entries. To avoid issues, refer to the content type change management guide for best practices on updating content type schemas without impacting live content.
To edit a Global field, log in to your Contentstack account, and perform the following steps:

Note: Changing or removing a field in a Global field increases the version number of all entries where the Global field is used.
Warning:
To edit a Global field via API, refer to the Update Global Field API request.