There's a couple ways you can decrease influence. Paint using the replace option with a value less than 1. You can also paint using the add option with a negative value.
Be careful with just decreasing weights like this, as if you remove them from one bone they get distributed to others, up to the number of influences you specified in the initial bind, and not always to the bones you expect or want. The easiest way is to simply paint in more influence on the bone you want to have more…