There are two ways of doing this, depending on whether you want to be able to preserve the original color or not.
Method 1: Use HSB
Click on the "R G B" label in the color picker, which will toggle the color picker between RGB and HSB:
Method 2: Add multiple fills
Click on the "+" button on fill to add a second fill.
By default, Sketch will select this fill to be a gradient. Change this to solid.
Select white as the color and adjust the opacity appropriately.
The result is a lighter version of the color you started with. The advantage to this method is that your original color is preserved as a fill on the object, so it's easy to go back and adjust, and you can blend different colors other than white or even gradients. Note that you can also reorder the fills with drag and drop.