Go to RubyForge and copy the URL of Redmine version you want to upgrade to.
Download it to your server using wget command.
Go to Redmine Site to check the requirements. Make sure you have all the required versions of gems installed. Please note that
Rails 2.3.5 will fail with RubyGems 1.5.0 and later, stick to previous versions of RubyGems
i18n 0.4.2 is required for Redmine >= 1.0.5 and i18n 0.4.2 require RubyGems 1.3.5 so You need to install a version of RubyGem between 1.3.5 and 1.4.2 for Redmine >=1.0.5
Uncompress the newly downloaded program archive in a new directory.
Copy your database settings-file config/database.yml into the new config directory.
Copy your email settings-file config/email.yml into the new config directory.
Copy the RAILS_ROOT/files directory content into your new installation.
Copy the folders of your installed plugins into new installation directory
Run the following command from your Redmine root directory:
VERY IMPORTANT: do NOT overwrite config/settings.yml with the old one
Go to your new redmine directory, then migrate your database:
rake db:migrate RAILS_ENV=production
If you have installed any plugins, you should also run their database migrations.
rake db:migrate_plugins RAILS_ENV=production
You should clear the cache and the existing sessions:
Check for any themes that you may have installed in the "public/themes" directory
Restart the application server.
Finally go to "Admin -> Roles & permissions" to check/set permissions for the new features, if any.