The simplest way to change a color used many different times in a project would be to have set up your artwork using "Global Swatches". When creating your Gradient- as you decide on each color click the + button at the bottom of the Swatch Panel to Create New Swatch- then click the global checkbox.
This will allow you to change this color anywhere in your document (whether selected or not) by simply double clicking on the swatch and modifying the color.
Chances are you probably did not set up your document this way- but it is a good habit to get into for this very reason.
Another option is to Select all the objects that you want to change color. Then go to Edit> Edit Colors> Recolor Artwork.
This will open a Panel (with a lot of different options).
Click on the Advanced Options
You will see a list of your Current Colors
Highlight one color and move the sliders below to change ther color across everything that is selected.
The Recolor Artwork has many more options than I have indicated and is worth looking into more as it is powerful.
All in all, setting up your project using Global Swatches is a better way to set up your construction for future projects.