If you mean the post name field, then we can update it for a post with:
wp post update 123 --post_name="new-slug"
where 123 is the post ID.
If we then try to update another post with the same post name:
wp post update 321 --post_name="new-slug"
then it will be set to new-slug-2 in that case, as it needs to be unique. If we continue this with other posts, then we will get new-slug-n where n is a positive integer.