Welcome to the board.
If you want to retain 25,000 in the cell but display only 25, then a custom format of #,##0, should do it. Note the trailing 0, this essentially divides the displayed number by 1,000. If you wanted to display 25 million as 25, you'd use 2 trailing commas.
If you want to actually change the underlying value from 25,000 to just 25, you would need to use a worksheet change event in VBA.