The variable products have not a “real” price since it depends on the prices of its variations; therefore to change a variable product price, you should change the prices of its variations.
You can do it through the Bulk Product Edit page by selecting “Include variations” and clicking on “Get Products”.
To select only some specific product variations (ex. only red variations), make sure you select the Type to Variations in the filters. Read more about how to properly use the filters here.