There are a few ways to do it.
You can use the TCE tool in Audio Suite: Audio Suite>Other>Time Compression Expansion
You can use the TCE trim tool: Click and hold on the Trim tool and select TCE. Then you just "Trim" the beginng or end of your region and it will make it longer/shorter.
You can use elastic audio: Enable the correct EA algorithm - for vocals I'd suggest Polyphonic. Then bring up the elastic properties region (Alt+5 or Region>Elastic Properties) and enter in a TCE factor. >100%=slowing down <100%=speeding up.
If I had to pick one of the above, it'd probably be the last one, elastic audio. If you use either of the first two, do not make incremental changes, it will continue to degrade the audio quality. Instead, make the change, if it's wrong Undo it. Then redo it, etc.
My personal preference would be to re-record the section at the correct tempo - this will yield the best results. All of the above methods will introduce artifacts which degrade the audio quality.
Hope that helps.