One way:
Assume you want to increase by 10%. If you want to do this in another
cell:
B1: =A1 * (1 + 10%)
or, equivalently
B1: =A1 * 1.1
To do it in-place:
Enter 1.1 in a cell. Copy it. Select your cell(s) to be increased.
Choose Edit/Paste Special, selecting the Values and Multiply radio
buttons. Click OK.