Q: How do I update my Mac OS X 10.4.11 to Mac OS X 10.5

Mac OS X 10.5 is not available as a download. It is only available as a disk, though it has not been commercially available for some years.  You might contact the Apple Store and see if you can obtain it here.  And by the way,  going from 10.4 to 10.5 is not considered an Update in the same sense that going from 10.4.2 to 10.4.3 is.  It is a change of operating system.

Hope this helps

If you can't locate a copy of 10.5 - you can buy 10.6 (check your Mac meets the specs) which will take you directly from 10.4 to 10.6 - and it's only around $29.00.

If you do decide to try to get a copy of 10.5 be very wary of buying from someone other than Apple - it has to be a full retail installer disk, not a grey disk which are machine-specific.

As the others say.

Here is some info from TexasMacMan which may help regards 10.5;

Mac OS X 10.5 Leopard installation system requirements

http://support.apple.com/kb/TA24950

Leopard is no longer available at the Apple Store but may be available by calling Apple Phone Sales @ 1-800-MY-APPLE (1-800-692-7753).

If you can't obtain a retail install DVD from Apple, look on eBay or Google the installer part numbers to possibly find at an on-line store. Here's what to look for:

MB427Z/A  Leopard 10.5.1 install DVD

MB576Z/A  Leopard 10.5.4 install DVD

MB021Z/A  Leopard 10.5.6 install DVD (single user)

MB022Z/A  Leopard 10.5.6 install DVD (5-user family pack)

Installing Mac OS X 10.5 Leopard

http://support.apple.com/kb/HT1544

Mac OS X 10.5 Leopard Installation and Setup Guide

http://manuals.info.apple.com/en/leopard_install-setup.pdf

After you install the base 10.5, download & install the 10.5.8 combo update at http://support.apple.com/downloads/Mac_OS_X_10_5_8_Combo_Update

Be aware that the cheapest you're going to get it is around $130 (if Apple still have stocks). From Amazon etc. it will be more.

If your Mac meets the specs for Snow Leopard, as Paul says, it's cheaper to go to to 10.6 which is still available from the online Apple Store.

Note that neither Leopard nor Snow Leopard support Classic mode, and SL particularly may require updates to printers and scanners as well as third-part software.

Before spending any money, please do "About this Mac" and find out your processor type. If it says, "PowerPC" you can install no higher than OS 10.5.8, AND you need a min 867 gHz processor and 1G RAM. If "About this Mac" says "Intel," you can install at least OS 10.6.8 Snow Leopard and possible OS 10.7 Lion.