how to reset user password...

I reviewed the information about my desktop profile, I should have administration rights. I tried to use my password to run the system updates and it did not work. How do I acquire the administration password? Thank you.

I bought a dell netbook from a co-worker and removed her information. Upon reviewing my profile, I should have administrative rights to run the software updates, but it will not accept my profile password for it. Is there any way to acquire the password, work around it, or reset it? Thank you.