1. 下载最新版 https://www.redmine.org/releases/redmine-4.2.3.tar.gz
2. 解压 tar zxvf redmine-4.2.3.tar.gz
3. 复制3.x的files至redmine-4.2.3
4. 3.x数据库备份 https://www.redmine.org/projects/redmine/wiki/RedmineBackupRestore
5. 新建数据库 redmine4
6. 恢复3.x的数据至数据库redmine4
7. 使用rvm或rbenv升级ruby至2.7.0
8. gem install bundle
9. bundle install --without development test
10. [可选] bundle install --without development test rmagick
11. [可选] bundle exec rake generate_secret_token
12. bundle exec rake db:migrate RAILS_ENV=production
13. 下载4.x对应的插件放到plugins内
14. bundle exec rake redmine:plugins:migrate RAILS_ENV=production
15. 启动

问题：

LoadError: dlopen(/Library/Ruby/Gems/2.6.0/gems/nokogiri-1.11.7-arm64-darwin/lib/nokogiri/2.6/nokogiri.bundle

gem uninstall nokogiri
bundle config set force_ruby_platform true
bundle install

下载太慢

bundle config mirror.https://rubygems.org https://gems.ruby-china.com

gem sources --add https://gems.ruby-china.com --remove https://rubygems.org/