I know this is a dumb question, can I update my OS?

I bought my machine from someone off of craigslist and I don't have any experience with the technical knowledge needed to build one of these computers. I'm slowly starting to get compatibility issues with running 10.12.6 and various software updates. I'm concerned for what the future holds, inevitably I either have to get a new computer or try to update. I know this is a really broad question but it's not as simple as just updating to a new OS, right? I was told never to do that by the guy I bought this from. Looking for advice or a direction to where I should start with this process. Thanks

submitted by /u/marubro
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