The problem is that all my projects are in unity editor 2019 version and if I switch to a newer version I’m afraid it will start asking me for api changes and other changes that it will make to my projects. What version should I install so it will not change too much my projects and will not damage them ?
Either use one of the LTS ones or - if you’re feeling really risky 
- the latest version (2021.1.15f1). It’s up to you and your needs really. Do you need a fairly stable Editor? Go with LTS! For the latest fancy tech, go with 2021.1!
(There may also be Alpha and Beta versions available, but do not use them for production. They are meant for testing and giving feedback.)
However: Make sure to backup your project first before you upgrade it! This way, you can easily go back if you indeed face issues.
I’d say it will also depend on what other assets you are using. Some of the content creators are updating their packages due to changes in 2020 and higher, so if you are using assets that have updated versions and you can’t upgrade them easily, I’d stick with the LTS version of 2019 (looks to be 28f1).
But as Mauri said, always backup the project before undertaking anything like this.