First read this, which tells you where your file associations are (what editor/program opens them)
Then, in your case, you'll probably need to first remove the *.fla file type, and then add it back in. When you do so, you then add the associated editor, and choose CS5 (external program, and browse to the CS3 startup file).
Let us know if this works for you.
Great, I've found the file associations before I wrote here, but I was kinda afraid to remove the default associated editor, it sounded so official... Now I removed it and added my Flash Professional CS5 as external program. Thanks it worked as a charm.
And what about my second question ? Is it possible or I have to go through Flash Professional to publish fast ?
I'm not certain - I haven't done that. But I found this link...
Part way through the page, it mentions this:
To perform testing or debugging in Flash Pro with a file you are editing in Flash Builder 4:
From the Flash Builder development perspective, choose Run > Test Movie or Run > Debug Movie. Note that each menu item has a Flash Pro icon next to it. Once the SWF window or debug session is closed, focus will return to Flash Builder unless there are compiler errors in frame scripts inside the FLA file associated with the project. Information about all errors is sent to the Errors panel in Flash Builder.
To publish the FLA file associated with the current project in Flash Builder:
From the Flash Builder development perspective, choose Project > Publish Movie. Note the Flash Pro icon next to the menu command.