updating rails app

update.rb

Upgrade Rails and gems

Update your Gemfile. Temporarily, set an exact Rails version, (e.g. gem 'rails', '= 5.2.1').
Run bundle update.
Relax the Rails version in your Gemfile again (e.g. gem 'rails', '~> 5.2.1').
Run bundle install.
Update your Rails config

Run rails app:update. This will try to write all configuration like it would for a new Rails app, and show you the diff to your current code.
Check the official Rails upgrade guide

Rails has official upgrade notes. Read through the relevant sections and make the appropriate changes.
Get rails console running

Fix all errors until the Rails console does not crash while starting up.
Ignore deprecation warnings for now.
Get rails console running with eager loading

Enable config.eager_load = true in development.rb.
Fix all errors until the Rails console does not crash while starting up.
Disable eager loading again.
Get rails server running

Fix all errors until rails server can render a page.
Click around a bit. Fix all errors you can find.
Fix tests

Fix unit tests.
Fix integration tests.
Fix deprecation warnings.

Run your tests and fix all deprecation warnings.