Hello there,
My friend give me the opportunity to practice my WordPress knowledge on his website (brave move! 
)
As I am new in WordPress world and my plan is to use one of the ThemeForest demos to change his website but I worried about old posts.
Will something happen to old posts or they are independent?
Thank you in advanced.
S.
If you are just updating the theme then they should appear but bear in mind that the formatting of the blog / posts is likely to alter with a new theme
I don’t want to update them but to install totally new one.
If it’s just formatting that is ok.
When you install a new theme, the posts/pages/categories etc should all still be there, it might just look a bit odd.
In case always back up before you make the change
Another quick note - for the most part you shouldn’t have an issue with a site that’s just blog posts since every theme typically has a blog. But if you have any custom post types (team members, portfolio, testimonials, etc) you could run into a problem if your new theme does not offer the same post types or if they use different naming.
Personally, we recommend taking a few extra steps before changing anything on your live site just to be extra safe. Specifically we think it’s best to take a full backup, test on a staging or localhost site, then once you’re sure updating your live site (here’s our guide to safely change your WordPress theme with more details).