Hi Pesh, welcome to AndroidPIT! 
As of right now, you should be fine with Android 6.0.1, as it should still run fine. Keep in mind that not only Android versions are important, but also the security patches. Sadly the Nexus 5 is not receiving any updates anymore, neither new Android versions nor any security patches.
It is possible that at some point some apps will not run anymore because Android 6.0.1 is too old for them, but the chance that that happens rather soon is pretty slim.
What you can do is switch to so-called Custom Roms. These are firmwares build by developers for the phones. With those, you will have a newer Android version on your phone. Here is a quick overview.
Btw, I also moved your thread to a different spot in the forum, as it fits better to the topic 