In Lightning, you can not simply edit the page layout of any object (Standard/Custom) from the detail page.
To edit the layout in Lightning please follow the below steps:
1 - Click on the gear icon and then Click on "Setup Home" which will take you setup page. 2 - In Quick Search box type "object manager" and Click on "object manager" which will take you to the list of All Object.
In Lightning, you can not simply edit the page layout of any object (Standard/Custom) from the detail page.
To edit the layout in Lightning please follow the below steps:
2 - In Quick Search box type "object manager" and Click on "object manager" which will take you to the list of All Object.
Also, below is the link of Trailhead module for your reference:
https://trailhead.salesforce.com/en/lex_implementation_customization/lex_implementation_customization_page_layouts?id=lex_implementation_customization
Let me know if this resolves your query Cheers :)
Thanks,
Amit Singh.
Make answer as best answer if this do the trick.