None of the answers get small tip about where get proper package version.
So my solution for debian 10 (buster) how to install not latest (1.20) but 1.18 version of kubeadm
sudo apt-get update && sudo apt-get install -y apt-transport-https curl
curl -s https://packages.cloud.google.com/apt/doc/apt-key.gpg | sudo apt-key add -
cat <<EOF | sudo tee /etc/apt/sources.list.d/kubernetes.list
deb https://apt.kubernetes.io/ kubernetes-xenial main
EOF
apt update
There is the most important thing I got a problem :). Get available package versions
apt list -a kubeadm
From the output select which you want to install
# apt list -a kubeadm
Listing... Done
kubeadm/kubernetes-xenial 1.20.0-00 amd64 [upgradable from: 1.18.13-00]
kubeadm/kubernetes-xenial 1.19.5-00 amd64
kubeadm/kubernetes-xenial 1.19.4-00 amd64
kubeadm/kubernetes-xenial 1.19.3-00 amd64
kubeadm/kubernetes-xenial 1.19.2-00 amd64
kubeadm/kubernetes-xenial 1.19.1-00 amd64
kubeadm/kubernetes-xenial 1.19.0-00 amd64
kubeadm/kubernetes-xenial,now 1.18.13-00 amd64 [installed,upgradable to: 1.20.0-00]
kubeadm/kubernetes-xenial 1.18.12-00 amd64
kubeadm/kubernetes-xenial 1.18.10-00 amd64
For me it was latest version for 1.18: 1.18.13-00
And now simply specify this version for packages
apt install -y kubeadm=1.18.13-00 kubelet=1.18.13-00 kubectl=1.18.13-00
Hope it could save somebody time and nervous