sakpolito,

First you need to make cuts, [T] key at play cursor, to isolate the part to be slowed. Next, if it is to slow down to less than half speed, you need to un-group the video from its audio -- select and use the little broken chain icon.

The change the video play speed there are a number of option depending on your program version eg realtime FX window in older versions or now the 'Effects' tab >> 'Video effects' >> 'Speed'. You can also simply drag out the end of the section on the timeline using 'Stretch mouse mode' which has a keyboard shortcut of [Alt+4].

Note if the stretch is quite long you will see the individual frame changes as each one is on screen for longer and longer. There are utilities available that actually create, with varying degrees of success and cost, new frames between the originals. 

Regards,

Carl