The case is easily opened by sliding your finger nail along the seam. Look for a tiny opening in one of the corners. That is the best place to start. On mine, it is on left front corner when the micro USB jack is facing you. Inside the battery and SIM (under battery) can be replaced. Mine also has a spot for a memory card (under battery). Inside the cover is a sticker with the SSID and WIFI Key (password). If the hot spot is unlocked, one changes the SIM to change carriers. I believe 3G refers to the technology of the network. Changing the SIM does not technology. I believe SIM contains carrier info as well as some info unique to the user's account.