currently we have asterisk server version demo 1.8 which is presenting 88% storage in / var. the question is what can I do or what command should I use to free up space
This is really a Linux question and not an asterisk question.
You need to find files you can remove and delete them, old logs, voicemails, call recordings, etc.
Linux command to free up space is rm comand (be careful )
or if they are important to you then (compress and) move them to some other storage.
the indox of the extensions are occupying space in the / var there is some command to erase those messages that have more than a week
You could write a script that uses the find command to locate files older than a certain date and delete them. Schedule your script to run weekly by using with cron.
Which command do you suggest?
cron? I’ve never used it,
since a month ago I am managing the Asterisk 1.8 server that we have in the company.
Due to the demands of the presidency, the possibility of migrating the current platform of asterisk 1.8 to the latest version is being studied.
I’m just taking back the installation of the Asterisk server with the latest version available 15.
If you tell me where I can get a manual to perform the installation of version 15. I would appreciate it
I suggest to use google for this non-asterisk things, and also to get involved in the Linux basic administration. I use google a lot 
I’d start here.
https://wiki.asterisk.org/wiki/display/AST/Upgrading+to+Asterisk+13
Asterisk 13 is the current LTS release of Asterisk, I would not migrate to Version 15.