To paraphrase something means to reword something written or spoken by someone else. Typically, paraphrasing is used in order to state something said by another without using their words exactly so as not to plagiarize.
Lines six through line ten, in Edgar Allan Poe's "The Cask of Amontillado" are as follows:
At length I would be avenged; this was a point definitively settled- but the very definitiveness with which it was resolved, precluded the idea of risk. I must not only punish, but punish with impunity.
The easiest way to paraphrase something to to state what teach individual line means to the reader. Sometimes using synonyms for words helps.
For example, the first line can be understood to mean: regardless of how long it takes, Montresor will have his revenge upon Fortunato.
The second part of the line, "this was a point definitely settled," refers to the fact that Montresor has already decided when he would enact his revenge upon Fortunato.
"But the very definitiveness with which it was resolved, precluded the idea of risk" refers to fact that Montresor knows exactly how it would go, and he was not concerned with any gambles he would be taking.
In the last line in question, "I must not only punish, but punish with impunity," Montresor is stating that punishment alone will not satisfy him. He knows that he will be able to escape from any consequences of his actions. Therefore, he is sure that he will be able to get away with murdering Fortunato.
In the end, a complete paraphrase of the excerpt is:
Montresor is stating the he will have his revenge upon Fortunato. He has already decided when he will do it. Montresor knows that he will not be gambling with anything and that he will be able to murder Fortunato without being caught.