Since you are on Windows, the best way is to use a hex editor. I personally use 010 Editor, but it costs money so the best free one is HxD:
https://mh-nexus.de/en/programs.php
You want to open the backup you made, and also open just the ART section, so you have 2 tabs open. You then place the cursor in the backup file tab to where you want to insert or replace the ART. Go to the ART tab, select all, copy, then paste it into the backup tab. HxD will overwrite all bytes following the cursor with whatever you copied. If the section you want to replace is bigger than the new content for example, you can instead select the block to replace before hand. You can also do it if you know the range via “Edit → Select Block”.
After you have done your edits, save it as a new file and flash that with the CH341A.
Appending to the end:
Replacing section:
