Changing a previous Apple ID on my iMac 2009 model that I want to get rid of. How can i do this? Thanks for any help here,...

Hi everyone,

Recently I bought this iMac as second hand and found that the previous technician that worked on it (help reset and delete or transfered all previous files) had his Apple ID still on it.

I tried to create my Apple ID under other users but when it comes to updating the Apple Store for upgrades, it always goes back an Apple ID from the last guy that created it, it asks for password that i dont have access to and cant change the account. I dont have any start up disc or manuals for this iMac. Does anybody know how i can change this. I am new to this environment, have always used Windows. Please help.