When I enter a value into any invoice or bill the figures pre populate.

When I enter a value of any invoice or bill the figures in the value box pre populate. I have to back space and try to re enter the figure three times before it will stop automatically filling the box . For example, if I want to enter £74.75, as soon as I hit the 7 it will fill the box with £72.00. I back space to the 7 and it will fill it with another figure beginning with 7. It's so frustrating. How do I delete the cache?