Up/down arrows, or dragging up/down on the reader itself. An overlay appears stating the new speed.

Which would be a great answer, except that several other folks have made the same comment today, so I'm afraid it's my fault and not yours. I checked it myself; the hint line in the reader says "Hold to read · ←→ scrub" and nothing about speed control. You followed the instructions I published, which were wrong. Just fixed it; a discoverability pass is now at the top of the queue this week.

Thanks for flagging it.