Asking previous advisor for a better letter of reference I recently applied for a national scholarship at my university. My application was accepted internally as one of few to be sent to the organization hosting the scholarship. However, the people screening applications at my university were slightly concerned about one of my reference letters from my last advisor, and sent it to me to show me what he wrote (I'm pretty sure those letters are supposed to be confidential, but that's beside the point). A brief summary of the letter is that I'm only slightly above average, and 1/4 of the letter is about his previous research accomplishments. I can't change referees since my application has already been screened. My question is, can I ask my previous advisor to improve his letter of recommendation?
You can certainly ask, and might be successful, or not. It is impossible to say. But a better approach might be for the scholarship people to ask the professor for a better letter on your behalf, explaining the reasoning and perhaps the fact that his is an outlier. 

Having an advocate may be more effective in a situation like this. Another professor might also prevail upon your advisor, but it would be impossible to hide the fact that you'd seen the letter.