There is a button underneath the editor field for each translation: Change relationship. Click it, and it will open a set of three or four options.
- Leave as is
- This one does nothing, it just allows you to unselect any othe option if you change your mind.
- Create new post
- This will disconnect an existing translation from the current main post and create a new empty connected post. The existing translation on the target site will stay, you can keep it or delete it later.
- Remove relationships
- This is available only if there is already a translation. Similar to the previous option, it will remove the relationship, but it will not create a new post. You can see it as the untranslate option.
- Select existing post …
- This opens a second set of options with a search field and the ten last drafts, private and published posts from the target site. You can search for other posts.
- The post status is appended to the title in brackets. If there are multiple posts with the same title, the post ID is included too.
- Select one of these posts to create a relationship between the current main post and that post on the other site.
This feature is not available on the Add New screen ( wp-admin/post-new.php), only on the regular post editor ( wp-admin/post.php).
Make sure you saved all other changes on the page, because when you hit Save and reload this page, all changes are lost.