To change margins in Google Docs, you can use the page setup feature by clicking File and then Page setup, or use the ruler to make adjustments on a real-time preview of your changes. The mobile app allows iPhone users to customize their page margins with options for Narrow, Wide, or Custom settings, while Android users are limited to switching to a pageless view. You can also automate tasks such as creating templates and populating data using Zapier integrations with other apps.