A guarantee simply means that you promise it will be such and such, for example, that it will work as you expect or provide the result you are hoping for. For instance, a company will often say that "your satisfaction is guaranteed."
A the word guarantee could be used in place of warranty but generally warranty is not used in place of guarantee.
I hope that is useful. They are certainly very close to one another in meaning and everyone would understand what you mean if you were to use them interchangeably.
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