Learn Polish: Musisz akceptować siebie. - You must accept yourself.

While both mean yourself in this context, siebie is the stressed, independent form of the reflexive pronoun. We use siebie because the action of accepting is directed specifically at yourself as an object, giving it natural emphasis. Also, the short form się generally shouldn't be placed at the very end of a sentence, whereas siebie fits perfectly there.

What case is siebie in this sentence?

It is in the accusative case, because the verb akceptować (to accept) requires a direct object in the accusative. The accusative and genitive forms of this independent pronoun happen to look exactly the same: siebie.

Does siebie change if the subject changes, for example if I wanted to say 'He must accept himself'?

No, it stays exactly the same! A great feature of the Polish reflexive pronoun is that it works for all grammatical persons. Just as Musisz akceptować siebie means You must accept yourself, On musi akceptować siebie means He must accept himself. It always reflects back to whoever the subject of the sentence is.

Could I put siebie earlier in the sentence, like 'Musisz siebie akceptować'?

Yes, you can, because Polish word order is flexible. However, placing siebie at the end of the sentence is the most neutral and natural choice here. Putting it before the verb makes it sound like you are placing heavy, dramatic contrast on the word yourself (as in, it is YOURSELF you must accept, not someone else).