Remove the battery and count to 10 and then replace it.
i found this
You do not reset the scale. It only saves the last recorded weight of the user. So, if you weighed yesterday, it will put your recorded weight in memory. The next time you select your user number and step on the scale, it will compute the difference from your previous record and save your current weight.
and also this
I had the same problem with my Salter 987 scale. Pull the battery out for about 10 seconds and put it back in. Then push the scale once so it turns on. After the blinking stops, push it once more. The scale will now zero itself. After the scale has zero'd itself it may turn off after a brief moment. It's now ready to use normally. If you're standing on the scale when it's trying to calibrate to zero, it will assume your weight is zero. Don't stand on the scale after replacing the battery until you first complete the steps listed above.
Not sure if this is the correct manual, but may be close. Page 3 and 4.
Salter Housewares Body Fat Computer Instruction Manual Page 3