You can use the Post Grid Combo plugin to change “Post Excerpt” sources. This article will help you change the sources of “Post Excerpt.”
To begin, insert a “Post Excerpt” block.
Click on the “Post Excerpt” tab.
Now click the “Excerpt Source” button and choose your preferred option.
If you choose “Excerpt“, the block will show the Post or Page excerpt.
If you choose “Content“, the block will show the Post or Page content.
For Custom Fields:
If you choose “Custom Fields“, you will find some predefined “meta key“. You can use them or add a custom meta key to show your content on the block.