As I need to deal with the previous versions of Sencha, I am planning to remove the latest version of 'Sencha Cmd' on Mac 10.9.5. How can this be accomplished?
It installs under /Users/<username>/bin/Sencha/Cmd/<version>/ and doesn't remove previous versions from the Cmd directory - which by the sound of your post means you will likely still have your previous 5.x available.
Typically, the install adds the following lines to your ~/.bashrc file which you will want to double check and make sure that the version in the shell path matches the old version you wish to use:
export SENCHA_CMD_3_0_0="/Users/<username>/bin/Sencha/Cmd/5.x.x.x"
export PATH=/Users/<username>/bin/Sencha/Cmd/5.x.x.x:$PATH
That said, I noticed with Sencha 6 that a symbolic link named sencha was introduced in the ~/bin/Sencha/Cmd directory - presumably to avoid polluting your bash configuration in future versions. You may need to rename or remove this to avoid conflict.
Lastly re-source / restart your terminal window.