Redmine

1. backup database
2. backup files folder(attachment) & config files
3. confirm Plug-in, some Plug-in cannot work in latest version of redmine
4. uninstall old version of redmine
5. install latest version of redmine, and related software
6. restore database
7. restore files folder & config files
8. upgrade Plug-in, if necessary

That is a simple migration scenario, which can be used for not only same server upgrade but also cross multiple physical servers and operating system.
Just for your reference~

I want to install a new server, since the current server is installed on ubuntu 16.04.6 LTS which cannot be updated.
which operating system to choose? need instructions for a new installation of redmine.
I tried installing the latest version of ubuntu and installing redmine from the repository, it did not work to install.

You can continue to use Ubuntu, version 20.04.2.0 LTS, which can support until April 2025~

there is a detailed instruction for installing the latest version of redmine on ubuntu 20.04.2.0 LTS ???

I think you need two things：
1. Guide Book -> https://www.redmine.org/projects/redmine/wiki/RedmineInstall
2. Spirit of assiduous study

The spirit of diligent study can and does help.
But your instructions are just awful.
Is it ok to describe the installation step by step?
For ubuntu, these are the steps.
For CentOS, these are the following.
What components are needed
Last time, using this instruction, I installed Redmine, Apache, Maria DB through the repository and could not install Ruby.
For two years nothing has changed in the instructions.
Have you tried to install rudmine on the newly installed ubuntu from scratch ???
Write a normal step-by-step instruction.