There are few ways to make a laptop faster that are as easy and cost-effective as replacing your existing hard drive with a solid state drive (SSD).

So to that end I would like to replace the (I am guessing) 5400 RPM HDD with a SSD and watch it speed up considerably. I don't know where to start with this.

Do I replace the existing SSD or the existing HDD with an SSD (assuming I’ve understood correctly that my iMac has both)? Documentation is sketchy at best for this 2019 iMac 21.5-inch model.