Pharaoh Sheshonq (or Shoshenq1) I reigned from about 945 to 925 B.C.E. In the Bible, however, he is referred to as Shishak, rather than Sheshonq (or Shoshenq).
The reason is technical but simple. In Hebrew, the sounds “n” and “m” are what linguists call “weak,” and are sometimes dropped. This is especially true in proper names. For example, we know that the city of Gath was spelled Ginti or Gimti in Egyptian inscriptions. And Hebrew Makkedah was spelled Manqedah in Aramaic.
So it is no surprise that the Egyptian name Sheshonq became Shishak in the Hebrew Bible. The “n” has simply been dropped. As far as the shift from “q” to “k,” that’s just a matter of transliteration. Shishak would more correctly be spelled Shishaq, but Shishak is the spelling found in most English Bibles.