You are probably referring what I have outlined in red:
There's a knob to adjust the front (thigh area) seat height and a lever to adjust the back (butt area). Here's the link to the page I got the image from. Might be helpful regarding parts:
https://parts.lakelandtoyota.com/sho...rimLevel=18357
That said, I have not had issues with that part. Have you tried removing the cap covering the screw that secures it? If may just need to be tightened. My mom's Kia Soul had a similar lever and it got loose over time and stopped working until you retightened the screw. Thats where I would start.