updating

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.

If no ” Get Button “  Replace the https in the page's address to macappstore. Credit to Contributor @Niel

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

Make Time Machine Backup  is very useful and can be used to Revert to Previous Working macOS

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 required update or upGrades to 64 Bit Versions.

Your MacBook Pro can be upgraded as far as Catalina. The advantage of Catalina is that is is till supposed by Apple, at least for the time being, with occasional security updates. El Capitan is no longer supported and is somewhat antique even for web browsing. The disadvantage of Catalina is that it is 64 bit only, and you will lose your older 32 bit software.

Perhaps 10.14 Mojave might be a good point to take your Mac for the time being and to consider Catalina later. You can upgrade directly from El Capitan to either of these choices.

Roaring Apps is a good site to check the compatibility of older software with more recent systems.

Check too that your printer is supposed in whichever macOS you choose to as your upgrade.

Make sure that you have a Time Machine backup of your Mac.

You can get installers for older versions of macOS at How to get old versions of macOS but do the homework first.