We often hear about getting newer or updated PDF plans and the desire to replace existing ones without changing the work you have done on top of it.
This task needs to be done from the ArcSite user site (read about the ArcSite user site).
It is much easier to accomplish this task from a desktop or other device which exposes the file system.
Navigate to the user site at https://user.arcsiteapp.com
If requested, log in with your registered ArcSite credentials
You will be presented with a list of existing projects
Select a project
You will then be presented with all of the drawings/plans within the project
On each one, you will see an Update PDF option, select it
Navigate to your new PDF and select
Finish upload process and you are done