Hi there!
Any iMac with a 32GB SSD is paired with a large 1TB hard disk and is operated as part of a Fusion Drive. Is yours no longer operating as a Fusion Drive, or are you wanting to upgrade the storage?
If the former, I'd recommend you explore How to fix a split Fusion Drive - Apple Support.
If the latter, rather than opening the iMac, which was never designed to be opened by the end-user and doing so can cause damage to the iMac, why not use an external SSD? An external SSD as your startup disk can give your Mac additional performance for system responsiveness, apps, startup, and more. An SSD will make your Mac 10 to 40 times faster for storage, and in turn, make your entire Mac faster and more capable. To learn more, see Use an external SSD as your startup disk … - Apple Community.
Jack