A bit late for this but for others who this may happen to...
1. Use a small diameter drill bit and drill down into the centre of the screw a short distance.
2. Tap,with a hammer,the slighter larger size of a small diameter torx bit into said hole.
3. Then you'll be able to screw the screw out normally using a spanner on the torx head(usually 10mm?)
This will preserve the threads.
Before you do this find a replacement screw as the threads are quite fine.
When replacing use the method of 'steel into alloy' where as soon as they are nipped tight,you'll only need to tighten further by an eighth of a turn....leave it and walk away
Lots of riders over tighten these screws whilst forgetting that it's just a cover and nothing structural as it were.
.
__________________
The Suit:What would you consider to be your greatest weakness?
Me: Honesty.
The Suit: Honesty? I don't think honesty could be construed as a weakness.
Me: I don't give a f**k what you think.
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