If you are on a long term support version of Ubuntu, this means, Ubuntu 20.04 or 18.04 you should be able to get GMT version 6.1.1 from the UbuntuGIS PPA
To add the UbuntuGIS PPA to your list of repositories do:
sudo apt-get install python-software-properties
sudo add-apt-repository ppa:ubuntugis/ppa
as described here.
Then you should reinstall GMT and get the version 6.1.1
You might have to do the soft link again after reinstallation.