Its alway sensible to back up your files before you do any kind of up date or up grade
1Back up any data you wish to retain, even if you are not going to reformat the drive with the "Upgrade" option. You can't be too careful with your important data.
2
Begin the upgrade process by inserting a Mac OS installation disc for an OS newer than Mac OS Panther 10.3 (Tiger 10.4 or Leopard 10.5).
Step 3
Open the installation disc and double-click the appropriate (normally center and top) icon to begin the OS installation. Your first decision will be to choose "Archive and Install" (which saves your older system in a new "Previous System" folder) or "Upgrade" (which will reformat the drive resulting in the loss of all data). Study these terms at the Apple support site if they are not clear to you.
Step 4
Follow the installation steps to check the target drive with "DiskUtility," then continue. At the proper time, the installer routine will notify you that it needs to "Restart" and boot from the DVD itself to complete the installation.
Step 5
Choose "Restart" from the "Apple" menu to ensure that the final phase of the installation proceeds correctly. The installer program should be running the Mac after the "Restart" command, just as it did before you invoked it.