Nesting IF statements?

"Update" Worksheet is getting data from "Sum1" I'm trying to modify "Update" as follows

PART 1: If the cell is "0" remain "0"
PART 2: If the cell is a "positive number" remain a positive number.
PART 3: If the cell is a "negative Number" take the inverse of the
"negative number" and if that number is equal to it's "Column Row 1 Number"
OR greater than it's "Column Row 1 Number" Then that cell becomes "Column Row1 Number"
If the number is less than "Column Row1 Number" then remains that number.
Note: When PART 3 has been determined all PART 3 cells are returned to a Negative number.

Thanks for any help mark