Infos on how to exchange the HDD for an SSD as primary and only drive?

Well it's a bit more complicated than just swapping things about.

First I wouldn't take the working HD out. But instead add the SSD as a second drive! Here we'll alter the boot drive from the current HD to the SSD so you'll get the speed you are looking for. Here's the IFIXIT guide you'll need to follow: iMac Intel 21.5" EMC 2389 SSD Dual Drive Installation. Here we are giving up the optical drive for the SSD unit using this adapter frame: 12.7 mm SATA Optical Bay SATA Hard Drive Enclosure. That way you also hang onto the HD for your deeper storage needs

If you don't want to give up your optical drive then swapping out the HDD will require a few pieces as the HD in this system is a 3.5" unit and the SSD is a 2.5" unit so you'll need an adapter frame. You'll also need to replace the built in HD's thermal sensor your systems heat management system requires. Here we'll use OWC's in-line thermal sensor. As you need to buy it I've added the OWC frame you'll need.

• OWC In-line Digital Thermal Sensor for iMac 2009-2010 Hard Drive Upgrade
• OWC 2.5" to 3.5" Drive Adapter Bracket

here's the IFIXIT guide you'll need to follow: iMac Intel 21.5" EMC 2389 Hard Drive Replacement