If you have previously followed Damian Mee’s initial setup guide, you can simply run:
git checkout v0.21.1
git log
./configure CXXFLAGS="--param ggc-min-expand=1 --param ggc-min-heapsize=32768" --enable-cxx --without-gui --disable-shared --with-pic --disable-tests --disable-bench --enable-upnp-default --disable-wallet
make
sudo make install
bitcoind --version
sudo service bitcoind restart