pixel_player571310279 Hello, and welcome to the forum 🙂
You can’t upgrade the entire workshop building, but you can upgrade the individual manufacturing ‘recipes’ within it (the same applies to the other multi-production buildings).
So from level 75, you can upgrade the mechanical parts recipe, to produced a greater number. As you progress through the levels you have a greater range of goods you can produce, and each product can be gradually upgraded in production output to a set maximum.
Hope that makes sense and helps.