You can use the ROUND function to reduce any value by a specific number of decimal places.
In the process, the final digit, the rounding digit, is rounded up or down.
The rules for rounding numbers that Google Spreadsheets follows, dictates;
 If the value of the number to the right of the rounding digit is less than five, the rounding digit is left unchanged.
 If the value of the number to the right of the rounding digit is five or higher, the rounding digit is raised by one.
Google Spreadsheets' ROUND Function
Unlike formatting options that allow you to change the number of decimal places displayed without actually changing the value in the cell, the ROUND function, like Google Spreadsheets' other rounding functions, does alter the value of the data.
Using this function to round data will, therefore, affect the results of calculations.
Round Numbers in Google Sheets
Follow this example to round numbers in Google Sheets.

Open a new, blank spreadsheet and enter the following in cells A1:C1.
 25.255
 10
 252.55

Select cell A2.

Select the Insert menu, choose Function, point to Math, and select ROUND.

Select cell A1 and press Enter. Use the fill handle to drag across the row and copy the formula to B2 and C2. The rounded results appear.
The ROUNDDOWN Function's Syntax and Arguments
A function's syntax refers to the layout of the function and includes the function's name, brackets, and arguments.
The syntax for the ROUNDDOWN function is:
 = ROUNDDOWN ( number, count )
The arguments for the function are:
 number — (required) The value to be rounded.
 This argument can contain the actual data for rounding, or it can be a cell reference to the location of the data in the worksheet.
 count — (optional) The number of decimal places to leave.
 If you omit the count argument, the function rounds the value down to the nearest integer.
 If you set the count argument to 1, for example, the function leaves only one digit to the right of the decimal point and rounds it down to the next number.
 If the count argument is negative, all decimal places are removed, and the function rounds up that number of digits to the left of the decimal point downward.
 For example, if you set the value of the count argument to 1, the function will remove all digits to the right of the decimal point, rounding the first digit to the left of the decimal point down to 10.
 If you set the value of the count argument to 2, the function will remove all digits to the right of the decimal point, rounding the first and second digits to the left of the decimal point down to 100.
Round Numbers Down in Google Sheets
Follow this example to round numbers down in Google Sheets.

Open a new, blank spreadsheet and enter the following in cells A1:C1.
 25.255
 10
 252.55

Select cell A2.

Select the Insert menu, choose Function, point to Math, and select ROUNDDOWN.

Select cell A1, type ",2" and press Enter. Use the fill handle to drag across the row and copy the formula to B2 and C2. The results, rounded down with two digits to the right of the decimal, appear.
ROUNDDOWN Function Summary
The ROUNDDOWN Function:
 Is used to reduce a value by a specific number of decimal places or digits.
 Always leaves the rounding digit unchanged — never rounds it up.
 Alters the value of the data in the cell — unlike formatting options that allow you to change the number of decimal places displayed without actually changing the value in the cell.
 Affects the results of calculations due to this change in data.
 Always rounds toward zero. Negative numbers, even though they are increased in value by the function, are said to be rounded down (examples 4 and 5 in the image above).