how can i reset the admin password , ive bought a mac mini in a garage sale , and i cant update :-(

Too many unknowns!!

Does the mini have a built in disk drive for CDs/DVDs? That will give a clue to its vintage.

If it does have an optical drive, get a Snow Leopard DVD. Post back when you are ready with the disk.

http://store.apple.com/us/product/MC573/mac-os-x-106-snow-leopard

Unless the mini came with disks, and you have not told us that....

You can give this method a try:

How to Reset an Administrator Account's Password in OS X

Tom

How to identify Mac mini models