[help] Can't reinstall OS

Hey guys,

I bought MacBook Air two weeks ago of a friend and I wanted to do fresh install of the OS but when I click cmd+r on the startup it shows black screen with big ass lock asking for password.

My friend tried to type the passwords he thought he knew but none worked! The classic mistake of typing unique password you didn't use somewhere else! He also don't have the receipt as he bought it like 5 years ago!!

It's not actually that bad it's working fine, a bit sluggish maybe but I'd just like to the fresh install to just because I couldn't do anything other than creating new user!

Any help on this or someone with experience can suggest how to do fresh install of the OS?

Alternatively, any suggestions/ideas on making this Mac like new?