how to change 0's to -'s for a selected block of cells

If you only want to change the formatting, not the content, of the cells you should look at the Accounting format type, but with no symbol selected.

All cells with a value of 0 will be displayed as - while keeping the value 0 in the cell.

But Accounting does introduce some "special" spacing in to the cell. So instead I'd suggest using a custom format. Custom formats can be split in to up to four sections, divided by a semi-colon, meaning you can define a format as such:

Positive values;Negative values;Zero values;Text values

So, if you happen to need a percentage to two decimal places, but replacement of zeros, you could do:

0.00%;-0.00%;"-";@

Where "-" will entirely replace all zero values and @ will display text as-is.
Make sure you remember to prefix the second value (negative numbers) with a - symbol!

If you don't have a format string handy for the positive and negative entries, note that you can obtain the format string for any standard type by first selecting the desired type on the format page and then moving directly to the custom view.

To implement:

• Select all the cells / rows / columns required.
• Right-click and choose Format Cells...
• Select Custom (or maybe Accounting)
• Enter the format string required