Update Macbook Air (mid 2011) with 10.7 Lion to El Capitan 10.11 ?

Hello,
my wife has an "old" Macbook Air 11" from mid 2011 (Core i5, 2GB RAM, 64 GB SSD) running OSX Lion.
We just used it for writing and updated it regularly (security updates) until two years ago.
Then we didn't need the machine any more - it slept (just loading the battery every now and then). And there were no more updates available (via Appstore).

Now we would use the machine again as it is in an overall nearly perfect condition. The battery was replaced some weeks ago, and it is running fine.
But: there is a lot of old software from 2012 installed: firefox, safari, openoffice, iwork 09 etc. -> everthing that is not upgradable.
And: Support from Apple has been discontinued last year.

I've found out (Apple support pages): the "newest" OS X that's running on it is El Capitan, 10.11.
Download of 10.11 installer (app) has been completed from Appstore; it says the Macbook is compatible...

==> Would You recommend me to upgrade the Macbook to El Capitan 10.11 ?
Would be a completely new install better ? How to manage that ?
Or are there other aspects or negative experiences ?

Thank you for some hints.
(and please apologize my English - I'm German)

Upgrading is a good idea and a clean install would give you a good start once you've backed up all the important stuff from that MBA. You can create a installer on a USB disk using DiskmakerX to boot into. Once there, you can erase the contents of the MBA's storage and do the clean install.

You can actually upgrade up to macOS High Sierra on this MBA and that's likely a better option than El Capitan.