Step 1.) Upgrade to Debian 11 first
The process to go to Debian 12 is not as smooth as 11, when trying to upgrade from Debian 10. In fact, it doesn't work directly, so you'll first need to follow this guide to update to Debian 11, reboot and come back here if successful.
Step 2.) Update sources.list
Update your /etc/apt/sources.list like this:
deb http://........
How To Compile QEMU Manually (using sensible options)
1.) Download the QEMU source file you want.
wget https://download.qemu.org/qemu-4.2.0.tar.xz
2.) Extract The Source File
tar -Jxvf qemu-4.2.0.tar.xz
3.) Switch to the extracted source
cd qemu-4.2.0
4.) Make sure we have the right libraries and tools to compile QEMU manually
sudo apt install build-e........