how can i update the software on my iMac 21.5 /2.9QC/2X4GB/1TB/GT750M

What is your Macs year and model? Go to apple logo in menu bar>about this Mac. If it is a Mac from the 2009/2010/2011 era the chances are high that High Sierra is last stop for this Mac. If it is a newer Mac, try to hit the software update button in the about this Mac screen

Assuming you have a 21.5" Late 2013 iMac, you can upgrade to macOS 10.15.7 Catalina.

You can get it here: How to download and install macOS - Apple Support

Be aware older 32bit apps will not run in Catalina.

Upgrading the OS is good for software compatibility but generally doesn't fix slow performance.

It sounds like you have a 1TB HDD drive, so I would instead suggest purchasing an external USB 3 SSD and reinstalling MacOS onto that as your new startup drive as a way to improve performance.

Almost any SSD will work. Otherwise see: Use an external SSD as your startup disk … - Apple Community

I don't know if updating macOS will resolve your speed issues.

If the iMac has been "work[ing] really slowly recently" but was working fine, using the same applications previously, that raises the question of what has changed.

You may want to go into System Report and check the S.M.A.R.T. status of the internal drive – just to check that it is not Failing.