Problems with viewfinder

Dear camera expert :)
something happened to the viewfinder in my 40D. i think that something is blocking the vision, the top of the viewfinder (on the whole line is kinda blurred) unlike other edges (left, right and bottom). i compared it with my friend's 40D, after removing the lens we noticed that in mine this soft foam/sponge is slightly longer. i quite certain this causes the obstruction. my question is, how do we fix it? can i do it in any camera repair shop or it is better to send it to canon? it is still on warranty.
Thanks a lot for help!