how do u change a admin password of a previously owned ibook

Take a look at this:

Change or reset a user account password on your Mac - Apple Support

Is the portable really an old powerPC based G4 or G3 iBook (white) model or is it a white MacBook?

If the computer is a MacBook (intel based later model from 2006 to 2010) some items will be different.

If the computer included the original install-restore OS X software media discs,

you may use disk #1 to try and reset the Admin password by starting the Mac

from the reduced version startup section on that system DVD.

Five ways to reset a lost admin password:

http://tidbits.com/article/14437

How to reset lost os X account password:

http://www.macosxtips.co.uk/index_files/reset-lost-os-x-account-password.php

Insert the disk #1 into the computer and hold down the C key on restart, keep

holding it down for a time (sometimes I wait until screen changes to a window)

and then 'do not run' the Installer, as that is a result of usual routine sequence.

Get past the first window choices to use the Installer window, its main menu

bar has drop-down choices; among these is Reset Admin password...

• Mac OS X Password Reset from Install DVD:

(note admin reset requires disc or command-line)

https://duckduckgo.com/?q=mac+os+x+password+reset+from+install+dvd&t=ffsb

See the link(s) from the search result, to best learn how to reset Admin Password

as there are two ways to try & do this. One is without install DVD media, the other

is with the install DVD media. These vary, too. So if you have the install DVD that

would be the better one to try. You can also choose to reinstall a completely new

OS X from the install-restore media; erase the entire hard drive by using included

Disk Utility in the running installer disc.

Is the computer really an iBook (G3/G4) white PowerPC & not an early Intel MacBook?

How did you identify the Mac? Does it start up or is it password locked in boot? If the

model is a recent build MacBook (pro, etc) it may not have been prepared for reselling

by the original owner of record, whose AppleID was involved in the present OS X install.

What you have does change the methods of performing the action requested.

Computers with OS X 10.6.8 Snow Leopard, with OS X install restore DVDs

can most easily use a system install restore DVD to access the reset method

in the Installer menu options. Later ones use Internet upgrade to OS X and

their systems are tied most directly to the owner AppleID accounts.

If the owner had prepared the unit for sale, or give away, the setup would be easier.

http://www.thesafemac.com/how-to-prepare-your-mac-for-sale/

• What to do before selling or giving away your Mac - Apple Support

However the links above would cover most contingency. Older iBook/PowerBook G4

model original install-restore DVD media is no longer available from Apple support.

Their last bootable system on DVD would be Leopard 10.5, depending on CPU speed.

Good luck & happy computing! 🙂