how can i update my imac 2007 because it only has os 10.5

That model iMac can run up to 10.11 El Capitan.

To upgrade from 10.5 you will first need a Retail 10.6 Snow Leopard Upgrade DVD that Apple no longer sells.

https://www.ebay.com/p/1071772191?iid=194223512928

Then you need to update that version of Snow Leopard to 10.6.8 using the Combo Updater.

https://support.apple.com/kb/dl1399?locale=en_US

Then you will need to go to the Mac App Store and upgrade to 10.11 El Capitan.

https://apps.apple.com/app/os-x-el-capitan/id1147835434?mt=12

However IMHO, it is not worth spending any money for RAM or Upgrade DVD for that old iMac.

If it's a Mid-2007 iMac you can upgrade up to El Capitan 10.11.6

How to get old versions of macOS - Apple Support

yeah because it wont update from 10.5 and it has 1 gb ram so if i just get another ram stick then it’ll update?

The current scarcity of proper 10.6 retail install disks is a far greater issue than adding RAM. When Apple stopped selling the install disk set a couple of years ago, first prices spiked to, in some case, more than the value of a used 2007 iMac, and now listings are few and far between. Amazon used do list dozens—now they show none available.

Disks MUST be the full retail version. Gray system install/restore disks from another Mac model may not work. Buying macOS disks on eBay has always been risky because some sellers advertise the disks as "full retail" but the disks that arrive are not, or are unusable burned copies. Then you get burned.

Upgrading to Snow Leopard only requires 1GB RAM. El Capitan will require 2GB RAM.

Hello, jabril1706.

The most recent version that your iMac 2007 can run will be macOS 10.11.6 El Capitan. That version was discontinued five years ago and no longer receives any sort of Apple support.

Because your Mac is only running 10.5 Leopard, you're only path to upgrading will start with getting your hands on a retail version of Mac OS X 10.6.8 Snow Leopard and getting that installed first. You can find Snow Leopard on the second hand market.

From there you can upgrade directly to El Capitan by following the links in this support document:

How to get older versions of macOS - Apple Support:

https://support.apple.com/en-us/HT211683

From one user to another, I wouldn't put any resources into that 2007 machine. It's way, way outdated and nothing you do to it can fix that. If you need a computer that you can use with the services and typical internet and job related work these days, you'll serve yourself better by finding a way to replace it.

But if this machine is just to play with as a hobby, then of course knock yourself out. Just don't expect to get very much out of it.