what operating system can i upgrade to from OSX 10.9.5 - macbook 2010 - 4GB, 113.66GB avail.

The specs as published show your build model (if correctly presented) to

be able to run the later High Sierra 10.13.x system, while memory (RAM)

may need upgrade to higher capacity chips of best specs to work well.

• How to upgrade to macOS High Sierra - Apple Support

https://support.apple.com/en-us/HT208969

If your MacBook (13-inch 2010) or mid-2010 MacBook/Pro both can run High Sierra.

The method to upgrade is presented ~ along with Macs able to run it.

PS: while there still may be active links to get an installer

for macOS Sierra (10.12.x) or Mac OS X El Capitan 10.11.6

from Apple servers, these could leave your Mac without

a fully functional Apple Mail.

When my mac desktop was upgraded past Mavericks that

it shipped with, to later El Capitan, that was a good thing.

For the most part, 10.11.6 has been stable. I've downloaded

but not installed 10.13.6 (you get installer, then get download)

yet. The file is in the Applications folder; it points to server.

Some who work around the security issue these older systems

may pose, can use Firefox browser and Thunderbird email client.

Other aspects of Mac system security of course, would not be

addressed by those third-party softwares; but they can work.

You may have to be sure your backup (Time Machine & other) is

available and current, then upgrade the system; this may involve

understanding how to use reinstall macOS from recovery.

Back up your data, check your applications for compatibility, and download High Sierra.

(176024)

10.13.6 (HighSierra) is the latest OS that runs on macbook 13" late2009 and mid2010.

But RAM easy to replace with 2 chips of 4GB (together 8GB), if you can still get them.

PC3-8500-1066MHz-DDR3-SO DIMM