Hi Hanson,
Thank you for your interest in Syncfusion Products.
Currently Essential DocIO do not have the direct support for modifying the bookmark name. As a work around, the bookmark can be removed and replaced with the new bookmark. We have created the sample to illustrate this. Could you please find the sample from the below link. Try running the sample and let us know if this helps.
Sample link:
F96916-46761120.zipPlease let us know if you have any questions.
Regards,
Poornima