Manually order related posts
When setting the related posts for a post you will have arrows show up in the "This posts links to" section:
This allows you to manually set a custom order for the posts you want to link to.
Then, in the block, widget or shortcode you use to display the related posts, make sure to set the order to "custom":
That will display the related posts in the order that you manually picked.
Take note that the ability to manually set the order is only available in the Gutenberg Block Editor at the moment.