editing slow motion videos in photos

Yes, I know how to use quicktime and iMovie. Apple implies that you can adjust when slow-motion starts and stops in Photos. "On your iPhone, iPad, or iPod touch, tap Edit or on the slo-mo video that you want to edit. On your Mac, just hover over the video. Then use the vertical white lines in the video timeline to set when slo-mo begins and ends." There is no video timeline except when you are trimming in Photos. My objective is to decide when to leave the slow motion in and when to play as normal.

Okay, let me add some clarification. I was trying to do this editing on my MacBook Pro that has been upgraded to High Sierra. I just opened up the same videos on Photos on iMac which is still running Sierra and the Slow Motion album is there with all the right controls and abilities I was looking for. And here I had done the upgrade because I thought this was a new feature. Go figure. Anyway, thanks for your help. — jw