How to add report number to articles already submitted to the arXiv

Go to the arXiv frontpage: http://arxiv.org, and login (upper right corner)
using username or email address and password. (arxiv will give you a new
password if you forgot your old.)

You will then be shown the list of articles you own. Note that your
co-author(s) may own some of your joint works. Click on the "Journal ref"
symbol next to the article you wish to add a report number to.

Enter Report number(*), and then click on "Add journal reference,
DOR, or report number".

You are done. The revised article will now appear as an "Article Submission",
and it will sit there for 1-2 days until the addition of the report number
has been registered by the arxiv.

(*) Several report numbers are allowed and should be separated with
semi-colons.