Do I need to clone it using ccc or super duper?
I use CCC, and it is easy to use and never caused me any issues.
BTW, it is always nice to have a spare bootable external back up drive around just incase. I never counted, but I am sure I have close to 20 bootable back up partitions mixed on various drives from over the years.
Sometimes is nice to have old bootable drives of old OS versions. For example, I have an old bootable version of MacOS 10.9 Mavericks because the version of Disk Utility has more features than newer versions.
In your case with having an old HDD, I would definitely have a bootable back up, as the HDD could start to go any day.
All that said, if you do not want to invest in CCC or something similar, you have other options that do not cost anything. For example, you can just connect an external drive using USB or TB, format it using Disk Utility, and install the version of MacOS you are using. Boot up the freshly installed MacOS on the external drive, and use Apple's Migration Assistant to copy all your files and settings over to your new OS disk.
Thank your your reply, My iMac is 3.4 GHz Intel Core i7, 8 GB RAM, 1TB HD, NVIDIA GeForce GTX 675MX
I use it for emails, MS Office and internet
How fast a computer feels can be extremely subjective. My Late 2012 iMac has more RAM, using an SSD, and the better GPU, but I am still surprised at your comment that your iMac is "working fine but slow" given that you use your iMac for MS office and web browsing.
I push my iMac a lot harder than you push yours with video encoding, playing WoW, etc., and I haven't replaced my iMac with a newer model due to it still feeling fast to me.
I would suspect that the iMac feeling slow probably has to do with mostly the HDD, and maybe it might be page swapping with only the 8GB of RAM. Since it would be page swapping with the HDD, this would seem extra slow.
What OS are you running? Maybe the version of MacOS is factoring in too. Mojave and Catalina do not play well with a HDD as the boot drive, and tend to run painfully slow according many threads I have read.
Either way, I am sure that just adding an SSD, either internally or externally, would be a huge dramatic difference for you. I would do that first, and for RAM, use the Activity Monitor app in the Utilities folder while doing typical tasks on your iMac to see if an upgrade of RAM would be beneficial for you.