If you have some Rails versions installed, you may create new Rails project with a specific version.
rails new command with version
You can do it with the following command.
$ rails _x.y.z_ new appname
$ gem list | grep rails ... rails (6.1.3, 6.0.3, 5.2.3 ...) ...
# Create Rails 6.1 project $ rails _6.1.3_ new rails6_1 # Create Rails 6.0 project $ rails _6.0.3_ new rails6_0 # Create Rails 5.2 project $ rails _5.2.3_ new rails5.2