I can't install OS X Mavericks because I have forgotten my 'administrator password'. I don't know what this is - how do I find it?

Hello everyone,

Perhaps somebody can help me?

Firstly, it should be noted that my MacbookPro works absolutely fine. However, I have recently downloaded an 'OS X Mavericks' update which is confusing me. In honesty, I don't actually know what 'OS X Mavericks' is, but it came through as an official Apple update so I guess it is fine. The installation is asking me for an 'administrator password' in order to install it onto 'Macintosh HD'. I know my Apple ID password and I also have a password which unlocks the MacbookPro after it has been in 'Sleep' mode for example, but neither of these work. I can't remember my 'administrator password' in the slightest.

What do I do?

Thank you in advance,

JBall