Hi,
I just began playing commandos and completed the first two missions. On the first one I got only 2/3 stars on time merits, and 3/3 on the second one. I would like to finish the game with every star so I can unlock the secret level (I know that it can be unlocked via password but I like to play that way). So my question is: can I replay a previous mission and get the merits "updated" or do I have to restart and replay at the end of every mission until I get the perfect score?

Thanks.

They've been trying to figure this out since the dawn of time. I've played levels slow, fast, killing, no killing, stealth, make a lot of noise....never worked consistently.

Good luck. :P

Yeah, I've never been able to figure it out either. That said, I've never had a problem accessing the final level, and I'm pretty sure I've rarely gotten more than one star on a level.

What secret level are you talking about? I couldn't find anything on Google by a quick look.
EDIT: OK I found a walktrough describing it. You need to be at least the rang of a captain. I never stumbled over this limitation yet ^^

I just played through the game and didn't get the final mission. Had to use the code to access it. I didn't really care about the stars since no-one seems to know how to actually get those.

I made a lot of noise, blew up barracks and shot guards so I guess that's a good way to not get stars :D

That shouldn't have any effect on getting stars; iirc there are only factors for how many stars are awarded each mission: time (three if you're under a certain amount of time, decreases the longer you take until you don't get any after a certain point) and damage to your team members (you get three if none of your men get shot/hurt at all).