I am unable to update the latest macOS after replacing my HD with SSD in iMac

First a note: iOS is the operating system used on iPhones. I assume you mean MacOS which is the operating system on Macs.

We can't see your machine, much less read your mind. We rely on you to give us enough information so that we can understand your issue if we are to help. We need a full description of the issue, information on what devices or machines you're referring to, the operating system (and version of it) currently on the machine. What you expect to happen, and where it fails. Err on giving too much information rather than too little.

So, for instance, why do you think swapping a HDD for an SSD would affect your ability to update? And update from what to what? and so on.

That, I'm afraid, is as far as that goes. The Next OS (Big Sur) starts with the iMac 2014...

macOS Big Sur - Technical Specifications - Apple Support (IE)

Again, what year is your Mac? What are you trying to update to?