In order to install a new (or different) version of Rails, you will first need to uninstall rails entirely with the following commands:
$ gem uninstall rails
If prompted for a specific version, select the one(s) you would like to uninstall.
$ gem uninstall railties
You may be asked if you are sure about uninstalling specific gems that rails is dependent on; you should 'yes' through each of these prompts.
Once you have uninstalled rails and its railties, you can then install the appropriate version with:
$ gem install rails -v 7.0.x
And run $ rails -v to confirm you now have the correct version!