The Samsung Galaxy S2 was released in May 2011, and the OTA update to Android 4.1.2 rolled out in February 2013. It was the last official Samsung Galaxy S2 update.
From the home screen, tap the Menu key.
Scroll to and tap System updates.
Tap Update Samsung software.
If an update is available, follow the prompts to install the update.
Otherwise, tap the Home key.
The handset is now up to date.
You can flash Lineage ROM and get Android 7.1.2 on that device (Galaxy S2x, i9100).
Personally, I'd chance it. Grab Odin, download the custom ROM, boot the phone in bootloader, then flash away.
Your device shows its age. If you brick the phone, you can re-flash a stock firmware image. If you somehow can't get it to work anymore, use it as a trade-in at Wow mobile for $50/$75/$100 depending on their promo, which is probably better than the money you'd get for selling that phone on Kijiji.
Consider this your opportunity to learn. You've got an older phone so you don't really have much to lose.
I've updated the Galaxy S2 LTE back in the day from Gingerbread to JellyBean using stock firmware through Odin (was easier than Kies). Odin requires windows (alternatives for OS X and Linux just don't seem to cut it), but gets the job done. Custom ROM's will get you much further on your phone.