Four ways:

1, try with rust remover, many can screw down.

2, there is a special screw tool (online), it can be used to open the hole drilling and other methods to take down the screw. For "one word" and "cross" head screws can be upright screwdriver, vibration screwdriver back (pay attention to the strength), and then twist.

3, if the screw is slipping, you can saw a word open slot deep. For hexagonal head screws can be directly vibration (pay attention to the strength), and then twist; if there is no prongs of the case, available saws or flat shovels artificially screw the head of the head slotted with a screwdriver twisted out.

4, slip wire or broken clear case, you can use a small drill bit to drill off, or you can directly saw off the screw.