Bitcoin Core switch to release version?
Start by stopping your running version of bitcoin core if you have one 

$ su - bitcoin

$ bitcoin-cli stop

Bitcoin server stopping

then install the last version

$ git clone https://github.com/bitcoin/bitcoin.git

$ cd bitcoin

$ git checkout v0.18.0

then compile it 

$ ./autogen.sh

$ ./configure

$ make -j $(nproc)

Now you can start it

$ bitcoind --version

Bitcoin Core Daemon version v0.18.0

$ bitcoind -daemon

Bitcoin server starting