[Greasemonkey] Replace html code

[MisterMeh]

Hello,
Normally I am just a user, no writer, but I want to make a little script:

On pimpero.mybrute.com I want to replace this line of the html code:

i=oy2%3A_cty2%3A_hy7%3Apimperoy2%3A_mi4298599g&k=4 5cf2b6c&amp

with this line:

i=oy2%3A_cfy2%3A_hy7%3Apimperoy2%3A_mi7586126g&k=5 3fd4b6c&amp

what do I have to write in the script?

Please give me the hole text, because I have no idea

I thougt this would be quite easy for you, it´s just replacing a line