First of all, please use this guidelines to make sure that you’ve updated to the most recent version of 2.6 branch. After that follow this guidelines:

cd your-redmine-location
svn update
svn switch https://svn.redmine.org/redmine/branches/3.0-stable ./
bundle update
bundle exec rake db:migrate RAILS_ENV=production
bundle exec rake redmine:plugins:migrate RAILS_ENV=production
bundle exec rake tmp:cache:clear tmp:sessions:clear RAILS_ENV=production