As someone who is not an expert in Excel, I would do it in this more or less brute force manner.
Starting with a column of numbers some of which are negative, use the ABS() formula for the first cell in an adjacent column:

Drag the handle at the lower right corner of the top cell in the second column to propagate the formula to all of the cells:

Select all the cells in the second column and right-click > Copy:

Select all the cells in the original (first) column, right-click and select "Paste Values": (the icon with "123"):

Delete the second column.
