Right steps to update a multiserver setup from ISPConfig 3.1 to the latest version

Hello,

I'm trying to understand what are the right steps to safely update my multuserver setup from v3.1.1 to v3.1.13p1.
I have read that it can be done this way:

But I have also read somewhere that if you modify for example postfix config files, then you need to do something special
for the upgrade scripts to preserve these changes. In my case many files have been modified, so I'm not sure what to do now.
Here is my setup:

How do I update it safely so that my modifications are not lost?
Thank you.

 

https://www.howtoforge.com/communit...nfig-from-3-0-5-4p9-to-3-1.74077/#post-350049

 

Ok, thank you.
1) But is my setup generally (installed features, etc.) compatible with the latest version, can I update it in one step?
2) About making custom modifications in ispconfig3/tree/stable-3.1/install/tpl: is this the right branch "stable-3.1" if I want to update to "3.1.13p1", or shall I look in "3.1.13" ?

And one more question:
3) Is it possible to add Roundcube if ISPConfig is already installed?