Cassettes are easy enough to change. All you need is a cassette lockring tool, and a chain whip or other means to keep the freehub from spinning as you loosen the lockring.
OTOH, cassettes aren't cheap, and if you're getting skip or auto shift because the chain is too wide, the cassette may be fine. In your shoes, I'd get a new chain first, and see how it runs, then decide about the cassette.
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