When you decide to expand the capacity of this industry click on the gold button in the upper left part of the production box. You will see an expansion dialog. You are informed of the cost of the expansion and asked to confirm or cancel. 

All industry expansions require lumber and steel. If you do not have sufficient materials for expansion you will be forced to cancel. When you have ordered expansion a build icon will appear in the production box and scaffolds will be placed around the building. Next turn the expansion will be completed. 

If you decide to use the lumber and steel for something else, expansion orders can be canceled by re-entering the expand screen on the same turn you ordered expansion.