The author uses the reflexive pronoun "themselves" here to emphasize that they were alone and that only Jesus, Peter, James, and John went up the mountain. (See: [[rc://en/ta/man/jit/figs-rpronouns]])
"Transfigure" means for a person's outward appearance to change. This can be stated in active form. Alternate translation: "his appearance had changed" or "he appeared very different" (See: [[rc://en/ta/man/jit/figs-activepassive]])