Trying to upgrade my car s sound system but don't know where to start?

I have a 2015 Honda Accord Base and I want to upgrade my sound system but I have no idea where to begin, I don't even know if I want/need an amp, sub, or just new speakers. I don't want anything to crazy expensive, is it possible to upgrade my sound for less than $500? Maybe even $400? Any kind of information or advice would help! Thanks!

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