new assignment - need help

hello all - I'm ivy. I've used excel for years but have a new assignment where my skills need to be kicked up a few notches. Hopefully this question is easily answered.

Excel 2010 - I need macro or vba code that updates cell value within a workbook to zero, when cell value is less than 1. I know I can write formulas within each individual calculated field to generate this change, but user wants to click a button to update all values at once.

What code would I use for vba or how would I record a macro to update an entire workbook?

Example: within cell range C4:L4 -> cells c4,d4,f4,g4,h4, and i4 the cell value less than 0. Need a macro button code to update these values to 0).

Thanks In Advance!