On your mobile, you can edit and re-share a post by tapping on the post from the calendar in the "Schedule" tab. This will open the "Post Details" where you can see engagements with the post by network. In the bottom of the page you will see the options to Duplicate, Reshare and Delete. Tapping on "Reshare" at the "Post Details" step will allow you to share your post once again without any changes being made to it.
Selecting "Duplicate" will take you directly to the "Customize" screen where you'll be able to edit the copy of your post and then share it. You can double tap images or text to edit them on this step.
From your computer, you can choose to edit your post by locating it in the calendar in the "Plan" tab, clicking on it and selecting "Edit". This will create a copy of the post and bring you to the "Customize" screen with the post ready for editing.
It's worth mentioning if you save your posts to your phone and then share to social media, you may have two copies of your post in the calendar - one that can be edited and one that is an "external" post and cannot be edited. Any posts that say "SENT" or "SAVED" can be edited. Posts that say "POSTED" will not be able to be edited, you can only re-share this post as is. For more information on external posts, check out this article: External Posts
Note: Editing and re-sharing your post does not edit past posts already on social media. To change past posts, you need to delete the post on the social media site and repost the edited version.