We have to keep our software updated for security and compatibility reasons. Our Rails systems are managed with RVM, so we can easilly ubgrade ruby. Upgrade Rails is easier because we are using bundler with Gemfile to manage our dependecies.
In local DEV environment should install new ruby and update bundle:

bundle update
rvm install ruby-4.0.1
echo ruby-4.0.1 > .ruby-version
rvm --default use ruby-4.0.1

In the server we should install new version and make it default:

rvm install ruby-4.0.1
rvm --default use ruby-4.0.1

We are using Capistrano and puma for deployment. From local, reinstall puma service: 

cap production deploy
cap production puma:install