Software Update

Take note regarding 64 bit apps and 32 bit apps as mentioned above.

You may find the following link useful in determining whether you can upgrade from El Capitan .

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

If you are not going beyond Mojave ( i.e. to Catalina or Big Sur )then you don't have to worry about the 64/32 bit app situation .

The Age of this computer will Govern what newer version of macOS it will Qualify to install. Use About this Mac from the Apple Icon on Desktop - the required Information will be there.

With the above information in-hand - follow this How to get old versions of macOS and verify what version this computer Qualifies to install. Make sure to use Only Safari to commence the download as Others may not work. Safari will open a redirect and open the Apple Store where you can commence the download. If an error message appears, that is probably because one is using another Browser.

Suggest using Go64 to find 32 bit application before attempting the upGrade. Some or many of the Application already installed may required update or upGrades to 64 Bit Versions.

Make a Time Machine Backup Before attempting. Notation - Mojave is the Last version to support 32 bit applications / extensions and Drivers. Catalina & Big Sur supports only 64 bit and there is no workaround.

Are there any AntiVirus, Disk Cleaner, Optimizers, Un-installers, etc installed which should be removed as per Developers Instructions. They are useless, unneeded, cause havoc and interfere with the normal operation of the OS and may even Corrupt the OS requiring a Reinstallation

You can be assured that updating or upgrading your operating system will not erase any of your other data. It is always a good idea to keep your operating system up to date as you will then have the latest features and security fixes. However, keep in mind that two things apply. First, how far you can upgrade the OS is dependent on the year and model of your Mac. Second, some of the newer operating systems will only work with 64 bit apps. Older 32 bit apps will then have to be upgraded, some at a cost to do so. So, educate yourself and do the needed research before upgrading. It can save you some frustration.

What you paper to have downloaded is an update file for Sierra. The update file is NOT the FULL INSTALLER File. As per link provided earlier - you can download the FULL INSTALLER