• Navigate to the site settings of the root website (within a site collection)
• Click Master pages and page layouts and locate the Page Layouts
• Make a note of the file name, for example: PageLayout.aspx
  Note: you will not be able to view or edit the page layout directly by clicking on the title or a menu item
• Using the example below, change the tenant and file name to create the required URL: https://tenant.sharepoint.com/_catalogs/masterpage/PageLayout.aspx?ControlMode=Edit&DisplayMode=Design
• Enter your new URL into a browser to start editing the embedded webparts and content.

If however you see this error: “A Web Part with this ID has already been added to this page” then take a look at this post first: https://www.sjlewis.com/2015/11/18/fixing-sharepoint-page-layouts-containing-webpart-with-the-same-id/