How do I Update OS for imac OS 8.6??

OS X isn't an update. It doesn't depend on the current operating system you're using. You can install it now if you want to.

Edwin is only partly correct. Your computer will probably require a firmware update that must be installed while booted directly into Mac OS. The firmware update must be installed before attempting to install OS X.

Your computer must also meet the other requirements of OS X: a minimum of 128 MBs of installed RAM (I would recommend at least 512 MBs) and a hard drive with a capacity of at least 10 GBs with 6 GBs free (I would recommend a 20 GB or larger hard drive.) There may be other concerns depending upon which specific iMac model you have.

If you need to use Mac OS then you will also need to upgrade to OS 9.1 or higher for use with OS X.

You will need to purchase a version of OS X for your computer. The current version of OS X is 10.4 which may or may not be installable on your iMac depending upon which one you have.

If your computer is old enought that it is running 8.6 (maybe 1999-2000?) then it may be difficult or impossible to install OSX because the hardware doesn't fit requirements. People have managed to get OSX running on some pretty ancient 'unsupported' machines so you could look around for info on that. If you have a coloured iBook you could look in here.

I would recommend simply getting a new machine if you can afford it. Much less hassle. If you can't afford it then you may have good success with installing OS9 which is still very popular. Try doing a search on auction sites like eBay to find an OS9 installation disk then download the updates (9.1, 9.2) from Apple's site.

OS X can be installed on all iMacs as long as they have sufficient RAM and hard drive space. Older iMacs require firmware updates.

OS X can be normally installed on all PowerMacs starting with the Beige G3s, all G3 and G4 Powerbooks except the original G3 Powerbook, all iMacs and iBooks.

Using a freeware utility called XPostFacto OS X has also been successfully installed on even earlier "legacy" Macs using the PowerPC processor.

Of course performance on older slower machines may leave a lot to be desired.

64 MB of RAM is not enough for OS X so get more RAM first! Then apply the firmware updates that may be necessary (see below) and you too can enjoy OS X!

http://docs.info.apple.com/article.html?artnum=86117

Some other considerations, like everyone else said, update the firmware before installing Mac OS X at all. Just attempting to install Mac OS X without the proper firmware installed before you install Mac OS X, may cause the screen to go permanently blank.

iMacs without Firewire can not install Mac OS X 10.4 and above, but can install earlier versions of Mac OS X if they have the proper firmware, and correct amount of RAM and disk space. The iMacs that have Firewire have two ports that look like the port in this picture that is second from the right

iMacs with Firewire but no DVD drive can install Mac OS X 10.4 and above if they get the Media Exchange Program CD version of 10.4 found on:

http://www.apple.com/macosx/upgrade/ 's right hand side if you are in the U.S. or similar link on foreign websites for Apple. If you are in that predicament, I can direct you to the correct page to get the Media Exchange Program Disk if you aren't in the U.S.

To tell which iMac you have, see this article, and we can help you determine how to best upgrade to Mac OS X and what options you have:

http://docs.info.apple.com/article.html?artnum=58669