If and only If the software update applet does not present an upGrade to something newer
You may need to manually download the newer version of macOS of Your Choice
How to get old versions of macOS which would include macOS 16 Sequoia
Safari will open a redirect and open the Apple Store where the download can commence.
Suggest having at least 50 GB of Empty Space on the drive.
It will allow the Full Version ( about 12.xx GB ) to download, then expand and then commence to install.
If there is Third Party Security Software installed - remove it.
It will interfere with both the download and installation process.
As previously mentioned and agree with
Always make a Time Machine Backup before doing upGrades