When tagging pdf files, you may discover errors or the client may wish to update content. Sometimes you can manage the update within Acrobat and sometimes you may need to have a separate document with only the updated pages converted to pdf, tagged, and merged with the main document.
Here's how to merge the files.
Create a copy of the file to work with.
Fix the errors in the source document, save, and convert to pdf.
In the converted pdf document, verify the page footer/headers are correct for numbering and orientation, verify the tags are correct and remove any pages you do not need.
Perform a File, Save As on this updated pdf file.
Open the pdf file where you want to insert the updated pages.
In the Tags tree, artifact all the container tags you do not need from the pages you will replace. You can use the Order tree and the Touch Up Reading Order (TURO) tool to place the items into the background as well, if the tag tree effort runs into issues.
Remove the parent tags of the containers you just performed the artifact on.
Add titles (through right-click, properties) to the tags just prior to and just after the location where you will be inserting pages allowing you to know where to place the new tags.
Delete the pages you will be replacing.
In the Page tree, drag and drop the pdf with the updated page(s) in the correct location.
In the tags tree, drag the new tags into position.
Content checks on the replaced pages includes ensuring the replacement pages:
In the Page tree, are set to use document structure.
In the tags tree, any links have proper alt text and link structure. Also ensure the links work.
In the tags tree, any Table of Contents links to the replaced pages work.
In the Bookmark panel, any bookmarks to the replaced pages work.