How do I unpair mouse and keyboard from my iMac?

Below are instructions to fix connection issues with Bluetooth mice and keyboards, you can. use them:

The following will require any USB keyboard and mouse. If you don’t have them, borrow or buy them, they are inexpensive!

1.     Connect the USB keyboard and mouse to the computer.

2.     Open System Preferences – Bluetooth

3.     Un-pair the wireless keyboard and mouse.

4.     Turn off Bluetooth

5.     Restart the computer in Safe Mode per the directions in Start up your Mac in safe mode - Apple Support

6.     Restart the computer normally.

7.     Open System Preferences – Bluetooth

8.     Turn on Bluetooth

9.     Re-Pair the wireless keyboard and mouse.

10.  Unplug the USB keyboard and mouse and begin testing the wireless devices again. 

Not much information in your post. All we know is that you have an iMac, running macOS Monterey, and you want to unpair your keyboard and mouse. You didn't include who manufactured them, so I'm going to assume that you have both an Apple Magic Mouse and an Apple Magic Keyboard.

*UPDATE: Looks like rkaufmann87 replied to you while I was typing my reply. He goes into more details on the steps you should take and I suggest you follow his instead of what I just provided you.

Since your Mac is running Monterey:

1. Go to System Preferences > Bluetooth
2. Find the Magic Mouse and Magic Keyboard in the list.
3. Hover over each, then click the “x” button that appears next to their names.
4. Confirm when prompted.

Important!

If these are your only input devices and you unpair both at once, you'll lose control of the system until you reconnect them or plug in a wired device. I recommend unpairing one first, verifying you have control via the backup device, then unpair the second.

To unpair the mouse and keyboard go to the System/Bluetooth setting pane and click on the i in the circle to the right of the mouse and keyboard.

In the next window click on the "Forget this Device" button.

These screenshots are from Sequoia but it should be somewhat the same with your system.