To truncate the number to say 2 decimal places use =TRUNC(Cell, 2).
Add a column, use TRUNC to create the truncated number and then Copy and Paste Special to get the truncated number into the original column. Delete the added column.
But usually (if you want to change the actual number, rather than how it is displayed) you will want to round, rather than truncate, so use = ROUND(Cell,2)