Are you talking about merely displaying the result to 5 decimal places or actually changing the value so that it doesn't have any more than 5 digits? If you're just displaying it, you can use the Increase/Decrease Decimal buttons in the Number group under the Home tab. If you're wanting to limit the result to 5 decimals, you could use the ROUND, ROUNDUP, or ROUNDDOWN functions. For example:
=ROUND(3.14159*RAND(),5)
The 5 at the end forces the result to be rounded to 5 digits or less.