In the dialog after you click publish there is a tiny folder icon. if you click that, an explorer window will open to a temporary folder. replace the previewimage.png by another one with the same size.

The Answer:
Click the Share/Upload button in Content Manager.
in the Image area click the little folder icon, in the new explorer window pass your image over the existing 'PreviewImage.png'
this will update the image in the Share Update window.

Same thing but different,
@Tailgunner - Now to send the update that allows the user to set a value in the text field and click a validate button. I will update the Visual Studios output to over right the subscribed version on my PC. Once tested do I open content Manager and click update?
I am sure that is correct but how will my isers get the update? do they get bothered with the update and have to accept it? Will thay need to unsubscribe and resubscribe?
Should I make a version 2 and submit it as new mod?

Steam will automatically fetch and install updates.

one more questions are there people out there that will test these update before I publish? how do I connect with them?

I don't think so. Mods are ususally banana software. Ripes at the customers' ^^ Just label it Beta. What are you working on btw?

I got sick of seeing Start money 0, new mod start money 10, through 1 million.
I wrote one with a dropdown and an XML editable values Set Start Money

Set Start Money

I need a code example how to create a length of water pipe, C#
I have looked at a few MODS, but for some reason everything I try fails.
I used the waterpipe tpo heat pipe but even that did not help,
I used the real world road convert, again could not make a road or a water pipe....

Singleton<WaterPipeAI>.instance.CreateSegment,
almost to simple