If you want to make Excel display a + and for positive and negative number values create a Custom Format

A B C < B format rule > < C format rule >
1 12 +12 +12.00 +0;-0 +0.00;-0.00
2 -10 -10 -10.00 +0;-0 +0.00;-0.00
3 7 +7 +7.00 +0;-0 +0.00;-0.00
4 -5.4 -5.4 -5.40 +0;-0 +0.00;-0.00
5 8.691 +9 +8.67 +0;-0 +0.00;-0.00

Select the cell(s), (or entire column/row), and open Format Cells, then select Custom and in Type enter format as required as shown above.

Note: if you’re using decimal places, values will round up or down as shown in example