Pictures of the window? What is the window made of? Operable or fixed? Double Hung, Casement, etc?

I’ll get a picture after daylight. It is a vinyl clad window. It appears double hung, not fixed. It is the top outside pane that is broken.

Short answer...you don't. If the windows are dual pane, they are part of a seal IGU (insulated glass unit) and must be replaced with a new IGU. That must be deglazed from the sash frame and replaced.

We’re not looking for new windows on an unheated shed

This is the window

Remove the interior glazing bead for the fixed upper sash, measure you glass edge to edge, order new sealed IGU with grids, once arrived...replace IGU.

Now...if you can find out who made the window, you might be able to source it from them as well. But there are online or local folks that can make that.