A little more detail would be helpful but I'll give it a shot. I'll assume you have a single .wav or .mp3 and wish to speed up one portion of it.
First you must cut out the section you wish to change. To do this place the start marker at the desired position on the sample by simply clicking on the marker bar across the top. Drag it to the exact location in the sample... it helps to zoom way in to get an accurate splice.
Next click on the sample you wish to split and press the T key on the keyboard. Whola! You've now spilt your sample in two. Do this at the beginning and end of the section you want to modify.
Now you have a stand along section of your song, you can now modify that section however you like including changing the tempo without changing the rest of the song.
Make sure you change tempo in the effects rack and not the master timing in the main screen. The master timing will still affect the entire arrangement.