Is it possible to replace the HDD to Sandisk SDSSDHII-480G-G25?

Here's the IFIXIT guide to remove your current HD: iMac Intel 21.5" EMC 2638 Hard Drive Replacement. Keep in mind this is a difficult system to open and its very easy to damage ones display panel it you don't use the correct tools and take ones time.

As to replacing your current HD drive with a SSD drive: Yes, it can be done you'll need to prep the drive up externally before you install it. You may want to talk with SanDisk support to ask them if they have tested their SSD in any of the newer 21.5" 'Thin Series' iMac's.