System Storage is at 65 + GB

Added >>

If going to High Sierra First >>>

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.

Choose High Sierra if it qualifies

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 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

Is this correct MacBook Air 13″, macOS 10.12 which is Sierra ??

If so, may suggest taking the upGrade step to High Sierra. It will better position the computer for any Boot ROM required during the High Sierra installation and Drive Conversion to the New APFS Format. The APFS would be needed during the Big Sur UpGrade.

What is “Other” storage on a Mac, and how can I clean it out?

Free up storage space on your Mac

OmniDiskSweeper safe to use

See used and available storage space on your Mac

And the Final Word from Apple on Managing the " Other "Category

• Other: Contains files that don’t fall into the categories listed here. This category primarily includes files and data used by the system, such as log files, caches, VM files, and other runtime system resources. Also included are temporary files, fonts, app support files, and plug-ins. You can't manage the contents of this category. The contents are managed by macOS, and the category varies in size depending on the current state of your Mac.

mistyv3309 wrote:

Trying to update my Mac iOS, but I don't have enough storage. I've deleted a ton of stuff but my system storage is taking up 3/4 of my entire storage...any ideas why this is so high?

What year is the MacBook Air 13″...

Update to what?

Can you verify your current macOS.... your byline says "macOS 10.12" Sierra

From the Terminal.app you can get a big picture view of GB , copy and paste:

diskutil list internal

please advise