Use a 30 Day Trial version of Carbon Copy Cloner Version 5 and clone your present Setup of the computer to an External Drive.
This Clone should be an exact Copy of the computer as it is now only on the extra drive.
Once that is done - Test that it works by shutting down the computer and have Only the Cloned Drive attached.
Restart and immediately hold the OPTION key until the Startup Manager appears and choose the USB Drive ( Cloned Drive ).
Once you have Tested that it Functions - go ahead and upgrade El Capitan to some newer hat this computer Qualifies to run
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. For Best results use Safari to commence the download as Others may not work.
Safari will open a redirect and open the Apple Store where the download can commence.
macOS Catalina 10.15 hardware requirements
macOS Mojave 10.14 hardware requirements
macOS High Sierra 10.13 hardware requirements
macOS Sierra 10.12 hardware requirements
Notation - Mojave is the Last version to support 32 bit applications / extensions and Drivers. Catalina, Big Sur & Monterey supports only 64 bit and there is no workaround.
Suggest using Go64 to find 32 bit application before attempting the upGrade. Some or many of the Application already installed may require an update or upGrades to 64 Bit Version
