Ensure your Vita has at least 50% charge before starting. A power failure during a firmware flash can permanently "brick" the motherboard.
Once you have your Psp2updat.pup file, you can install it using a PC and a USB cable. Method 1: Using Content Manager Assistant (CMA) Psp2updat.pup Download
Many users in the modding community look for specific versions (like 3.60, 3.65, or 3.74) to ensure compatibility with HENkaku or Enso. Ensure your Vita has at least 50% charge before starting