iMac 27-inch, late 2015 - What should I know about replacing my hard drive with a ssd drive?

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DO NOT do it yourself. iMac was never designed to repaired or upgraded by the end-user, and these modifications have great risk of damage or permanent failure to the iMac. Instead, use an external SSD.  For more info, instructions, and what external SSD to buy, please see: How to Setup and Use an External SSD as your Startup Disk on a Desktop Mac - Apple Community.

Jack

1. Do not attempt to do it yourself, have your local Apple Authorized Service Provider do the work. 2012-2022 iMacs are NOT intended to be serviced by their owners. They are extremely complex and extremely easy to damage. Please leave the work to a professional that has the experience, expertise and skill to do it right. Please refer to the following video that shows how difficult replacing a drive in your computer is.

Upgrade/Install a SATA Drive in a 27" iMac (2012-2019) (macsales.com)