Go to the uploader: https://gerrit-patch-uploader.toolforge.org/.
Amending an uploaded patch
To make a change to an existing patch, copy the Change-Id line from the Gerrit page, and add it to the bottom of your commit message.
For example, to add a new patch to https://gerrit.wikimedia.org/r/96693, fill in the patch uploader as usual, but add the following text to the bottom of the commit message field:
The Gerrit patch uploader will use the "Change-Id" to find the current change on Gerrit and add a new version to it (instead of creating a new change page).