The Stylo 3 is almost 4 years old now, and even the best manufacturers don't support their phones that long. If it's not detecting any official updates to install, your only option would be to look for a third party version of the OS. Some independent developers will continue supporting a device longer than the manufacturer, but even this isn't a sure bet. Even if you found one (which is unlikely for lesser phones), there are additional concerns. You have to make sure the specific OS is designed for your make, model, and version of your phone (meaning a Sprint Stylo 3 OS wouldn't work if yours is a Verizon version). You also have the inherent risks of installing an OS from someone you don't know, and the act of installing risks bricking your phone if you aren't extremely careful.
Given the age and low specs of the phone, I think it'll be more trouble than it's worth trying to update it, even if you can find an update. I would either leave it as is, or buy a better phone all together.