Is it worth to upgrade from 2.3.4.16 to 2.3.5 ? I dont see many chnages.
Always best practice to get to the latest version. If you don't have any issues that 2.3.5 clears up and it is time consuming to do the upgrade then hold off.
Whats involved in the upgrade? is it in place upgrade? do I need to stop the services before running? would it keep my settings?
https://support.symantec.com/en_US/article.TECH155523.html
Upgrades You can upgrade from 2.3.1 through 2.3.4 to 2.3.5.
Yes, the upgrade is inplace and it would preserve all of your settings.
Do I have to stop any services before the upgrade? or just run it?
Take a backup first. After that you can just run the executable and follow the on-screen instructions.