I suggest to use tftp only (faster to upload)
1.Check for the present IOS file (show flash:), and free space.
2.Take back up of the file to TFTP (copy flash:IOSFILE tftp:)
3.Delete the existing IOS file
4.Check for the free space(If the file does not get delete permanatly, use "Squeez flash:" )
5.Upload the new IOS using (copy tftp: flash:)
6.Reboot Switch, have console connected to monitor connected to just make sure every thing goes well.
7.Incase any problem use manual boot(using BOOT TFTP:oldIOS) to boot the swtich.
8.Fall back to the oldone.