How can I uninstall Ruby on ubuntu?

Why you are removing old version of the ruby?

rvm install 2.4.2 // version of ruby u need to insatll rvm use 2.4.2 --default // set ruby version you want use by default

Using rvm you can install multiple ruby version in the system

Please follow below steps install ruby using rvm

sudo apt-get install libgdbm-dev libncurses5-dev automake libtool bison libffi-dev
gpg --keyserver hkp://keys.gnupg.net --recv-keys 409B6B1796C275462A1703113804BB82D39DC0E3 
curl -sSL https://get.rvm.io | bash -s stable 
source ~/.rvm/scripts/rvm
rvm install 2.4.2 
rvm use 2.4.2 --default 
ruby -v

The installation step will change for different Ubuntu version

For more info,

https://gorails.com/setup/ubuntu/14.04

You can use sudo apt remove ruby