Hi,
Edit the view files product / show_quantity and layouts / quantity via the menu Display>Views
There, you'll find the pieces of code:
and:
which respectively define the increment and decrement steps.
Change them to use 5 and -5 and it should work like you want.
Alternatively, and without any view customization, you can set the minimum quantity per order to 5 in your product, and set its "quantity input" setting to "show select". This will display a dropdown with the values 5, 10, 15, 20 etc automatically for the quantity selection.