The save games campaign difficulty are made in H level. If you want to change that you have to make a small change using EsfEditor:
1. Open EsfEditor and select menu, File, Open Esf
2. Navigate to ...\napoleon total war\data\DMN\DMN_Packs\DMN_Nap_Saves. You'll see nothing so select All Filles in the right lower button.
3. Select the save you want (if you have not keep a backup do it now in this window with copy/paste, keep both).
4. Now go in CAMPAIGN_SETUP_OPTIONS and change the marked number to 0 for Normal or 1 for Hard. The default here is 2 for VH.
5. Save your... save from menu, File, Save (in EsfEditor save changes only like this. If you close it using red X and answer Yes in Save prompt nothing will be happened and you will have to restart Editor and the whole procedure!).
For Battle difficulty things are much easier: while in game press Esc to bring Options, go in Game Settings and adjust it.