Get latest from the store or here
https://wiki.debian.org/DebianTesting
Run the following as root. Change the value of LATEST to the latest stable codename
LATEST=bullseye
sed -i "s/$LATEST/testing/g; /security.debian.org/d; /-updates/d; /-backports/d" /etc/apt/sources.list
echo "deb http://security.debian.org testing-security main" >> /etc/apt/sources.list
apt update && apt upgrade
apt dist-upgrade