Change the HDD for a SSD

The new Thin iMac's are different still again from the older models.

Replacing the HD with a replacement drive is one thing (like for like). Upgrading to a different storage type altogether is another matter. First you need to deal with the thermal sensor and the then need to think about the mounting hardware (if you didn't get a unit with a HD to start with).

I would recommend going the a hybrid drive (SSHD) like Seagates' offering Vs going with a straight SSD. Presently you can't upgrade the blade SSD units as you can't get a third party SATAe with a M2 interface yet as the M2 standard is still being defined. As soon as it is I think you'll see a flood of options.

You may want to also review this answer: Can I add a SSD myself