I have a iPhone 4 and when I want download some apps it says update and its a. latest software

Unfortunately, your iPhone cannot update beyond iOS 7.1.2 which restricts which apps you can download.

If you are able to download the current version of an app on another device (like a computer with a recent version of iTunes), you may be able to install the older versions of apps you don't already have onto your iPhone 4. Install an app on an earlier version of iOS or OS X - Apple Support