There is no dedicated function to adjust brightness.
You can (however) do this by using the HSV color model.
Breaking HSV (Anachronism) into Parts:
H (Hue) Controls the Color itself.
S (Saturation) of the Color.
V is called Lightness or Value.
'V' is what you want to control.
Just set the LEDs color to a HSV value and change it's V value.
leds[i] = HSV(color, saturation, brightness);
While 'Color' and Saturation' remain the same (to keep the same color), you can change the 'Brightness' directly on the third parameter.
Note that 'leds[i]' is still defined as 'CRGB', not as 'HSV'.
This assignment uses an implicit conversion, that is defined in the library.
You can see the 'HSV' usage example within the 'Cylon' example on the FastLED library.
There you will find the Hue value that gets changed; but it is the same principle as 'brightness'.
Now there is a problem to solve.
The FastLED library can convert 'HSV' Colors to 'RGB' Colors; but not the other way round.
So you cannot simply get the 'hue' and Saturation from an 'RGB' Color with this library.
You have 2 Alternatives:
1. You can just define an extra LEDs array with HSV colors and synchronize both arrays to contain the same data (everytime you want to change a color, you would set a 'HSV' color in the second array, convert that array to 'RGB' by using 'CRGB(hsv_leds[i])' and save that in the first array, which get's used by the FastLED library).
This is relatively easy, but you effectively need double the memory for storing the color values.
This is not an issue, if you have enough memory or few enough LEDs to control.
2. You can write your own function to convert 'RGB' values to 'HSV'; then use it to access the 'hue' and 'Saturation' values for the targeted 'RGB' Color, create a new 'HSV' Color with it and the new 'Brightness' and use it to set the 'RGB' values for the FastLED library.
In this forum thread there is a brief discussion about that, though it leaves you with the implementation yourself.
I think there are suitable implementations, that can be found on the web.
A quick search gave me this code from github gist.
I have not tested it, so I cannot say, if it really works correctly, but you should give it a try.
I'm sure there are also other implementations.