If you really want a newer version of Android, you're only option is to install a custom ROM.
xda-developers.com has an active community for the S2. If you're new to romming, there's a bunch of guides and stuff on XDA's website (here's the link you want:https://forum.xda-developers.com/galaxy-s2/orig-development), and elsewhere on the web.
I haven't seen any Oreo ROMs for the S2, but there's a handful of Nougat ones.
I'll admit, romming has a learning curve, but you'll get the hang of it quickly.