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.

Change relationships

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.