Hi @iur-kaia!
It should be an additional run step added to your job, usually towards the start of the job run process. As an example, something like this:
jobs:
test-and-build:
macos:
xcode: 11.6.0
steps:
- checkout
- run:
name: Set Ruby Version
command: echo 'chruby ruby-2.6' >> ~/.bash_profile
- run: bundle install
...