Extract the scenarios and put them into your scenarios folder near the end of the list
Load each scenario and press Deploy for Japan as that is the nation you are playing as
Once the screen loads press this button and change the number of turns you are comfortable with
save the scenario back into the slot you saved it originally then do the next scenario. When you are done changing the scenarios you need to click on the campaign then press the Edit Campaign button. Click on the first scenario in the campaign and you will be asked if you want to clear or replace it.......you want to replace it with the version you just increased the turn then repeat with the rest. When you are done rebuilding the campaign with the lengthened scenarios press SAVE for the campaign.
You might want to make a copy of the game to do this on first