nowhere in my computer shows me software update. I've looked, followed the steps and I still can't update my computer.

Which system version? On mine I check the App Store.app. Note this will only show updates, not upgrades.

How to update the software on your Mac - Apple Support

Software update, upgrade--what's the difference? - http://support.apple.com/kb/HT1444 [No longer directs to a document discussing this but basically an upgrade is a major system release such as replacing OS 10.4 with 10.5, and an update is a minor release for a particular system, such as taking 10.4.8 to 10.4.11. Software Update will check for updates but not necessarily for upgrades, particularly if a newer system will not run on an older piece of hardware.]

Without knowing what version of Mac OS is installed you make it tough to answer the question. I have Big Sur 11.1 and can find it in System Preferences - Software Update. It is also in About this mac too which is located by clicking the Apple symbol in the upper left of the display.

Please read the following user tip and restate your question/problem in detail: 

Writing an Effective Apple Support Communities Question - Apple Community  

What Mac type, model ID and year do you have?

What system are your running currently?

What software do you want to update, the system?