Depending on the location and ease of access to the stripped Torx screw, removing it can be simple and short, or quite a chore to accomplish. A Torx screw is commonly used in applications to decrease the chance of cam-out, which is when the torque of the screwdriver exceeds the screw's capacity and the driver slips out of the screw. Removing a Torx screw is a trial and error process, so be patient and don't resort to force, because this can exacerbate the situation and make removal much more costly and time consuming.
Apply some penetrating oil, such as 3-in-1, Liquid Wrench or PB Blaster to the screw and let it sit for a few minutes before attempting to remove the screw again. The penetrating oil may remove the rust holding the screw in place, making it easy to remove the screw without much force. The longer the oil is allowed to soak, the better.
Use a rotary tool to cut a straight notch across the face of the screw just large enough to hold a flathead screwdriver tip. Use the screwdriver to back out the screw.
Flatten two sides of the screw head using an angle grinder, and try to remove the screw with a pipe wrench. Reapply the penetrating oil if the screw still won't budge, and drip some melted candle wax around the screw head. The wax helps lubricate the screw threads.
Use a blowtorch to carefully heat the screw head while avoiding any sensitive components in the vicinity. The heat expands the metal and cracks any material locking the screw in place. Reapply the wax and attempt to remove the screw with the wrench or screwdriver.
Drill the screw out using an appropriate size screw extractor bit, which you can purchase from most well-stocked hardware stores. Go slowly and follow the directions indicated on the drill bit packaging. Drilling the screw out may destroy the mount, making it impossible to seat a new screw. You also can take the part to a machine shop and have them remove it for a fee.