Two ways to undo an overclock like this: replace the modified jumper block/individual jumpers with a stock jumper block, or configure the modified jumper block/individual jumpers to mirror the setup of your original system.
I'll assume that you're using individual jumpers here. In your case, you'll need to locate pin 1 on the jumper block (SPEED/CTL is printed down the side of the block) - there's usually a little number 1 printed on the upper-left-hand side of the jumper block to identify pin 1. Then, use the instructions on that page to configure the top four sets of jumpers to reflect 300MHz - essentially, this would involve placing jumpers on sets 1, 2, and 3. Leave the others alone, as only the top four sets directly affect the processor frequency and the rest should already be properly configured.
These things take very small jumpers, like the kind used on old SCSI hard drives. I'm afraid I can't remember where to get more other than maybe Radio Shack.
I'll assume that you're using individual jumpers here. In your case, you'll need to locate pin 1 on the jumper block (SPEED/CTL is printed down the side of the block) - there's usually a little number 1 printed on the upper-left-hand side of the jumper block to identify pin 1. Then, use the instructions on that page to configure the top four sets of jumpers to reflect 300MHz - essentially, this would involve placing jumpers on sets 1, 2, and 3. Leave the others alone, as only the top four sets directly affect the processor frequency and the rest should already be properly configured.
These things take very small jumpers, like the kind used on old SCSI hard drives. I'm afraid I can't remember where to get more other than maybe Radio Shack.