need to update my os

From your Apple () menu > About This Mac, keep in mind that year and model in the ( ) of the product description line. Using the Safari browser, visit How to get old versions of macOS, and compare your mac year and model with the hardware requirements for the operating systems. Follow the appropriate hardware compatible link to the Mac App Store and download the full installer.

The last operating system to support 32-bit and 64-bit compatible applications is Mojave, which is now out of Apple's support window. Catalina and beyond only support 64-bit applications, and (printer/scanner) drivers. You may want to obtain a list of your existing 32-bit applications using the free Go64 application, and then based on that list, determine if there are any current 64-bit replacement versions, or printer/scanner driver support for the intended operating system upgrade.

Always make one last Time Machine backup of your Mac before applying any upgrade.

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

Tips Big Sir Upgrade Released Nov 2020

Tips Monterey Upgrade  Released Oct 25, 2021

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.

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

How to upgrade to macOS Monterey - Apple Support

How to get old versions of macOS - Apple Support