How to update OS X 10.5.8 to he latest El Capitan?

First check your iMac and see if you can.

OS X El Capitan - Technical Specifications

Are you sure your machine is eligible for update? El Capitan has high system requirements. It requires at least OS X 10.6.8. You should upgrade to Snow Leopard first. More info here: Upgrade to OS X El Capitan - Apple Support

You can jump to it from 10.6.8. I am not sure if your Mac will run smoothly after the update. Please consider checking system requirements and your machine specification.

What model and year is your iMac?

You may upgrade from 10.6.8. Requirement are at least 2 gb of RAM and 8.8 gb of free space on your hard drive(minimum system requirements)

How old is your machine? If you'll check the article carefully most likely you'll notice more requirements:

• iMac (Mid 2007 or newer)
• MacBook (Late 2008 Aluminum, Early 2009, or newer)
• MacBook Pro (Mid 2007 or newer)
• MacBook Air (Late 2008 or newer)
• Mac mini (Early 2009 or newer)
• Mac Pro (Early 2008 or newer)
• Xserve (Early 2009)

Not enough. 1 gb of RAM is not enough. You need at least 2 gb to RAM(aka Memory)

In "About This Mac", there should be a button the says either "System Information" or "System Profile". click it and find which model iMac you have and what year.

Not enough RAM. You need to upgrade it. Period.

You are welcome. Good luck.