Change it by clicking on the arrow next to the name of the clip, change it from oppacity into time remapping then click spped. If the arrow isnt next to the clip title the you will have to drop down the area in the main clip setting, change spped to 50% by dragging down line, you can also hold command and click on the line to leave pointers and drag the line in between if you wanted to only slomo parts off the clip. experiment.