One way:

Enter 1.15 in an empty cell. Copy the cell. Select the cells to increase
the value. Choose Edit/Paste Special, selecting the Values and Multiply
radio buttons.

In article ,
mrudnet wrote:

If I have an excel worksheet with numbers in multiple cells and I want to
increase every number by 15%, how do I do that across the worksheet rather
than go into each cell individually? Any advice would be much appreciated.