For updating golang, you need to first uninstall golang. Check where is the
which go. The output might be similar to:-
souvik:Development souvikhaldar$ which go /usr/local/bin/go
Delete the directory as is the output above.
sudo rm -rf /usr/local/bin/go
Now install the required version. Visit this link
Check the version by running :-
souvik:~ souvikhaldar$ go version go version go1.12.3 darwin/amd64