If you are trying to bright a section of clip then you may first trim the section and use the Lighting effect for desired duration. Please check the below help article on how to use Lighting effect:
You may also apply brightness effect or other lighting darkening effect to the clip and then keyframe that property so that the effect varies across the duration of the clip rather than being applied uniformly to the whole clip.
Keyframes for a given property can be set under Adjust Tab/Adjustment Palette/ and Applied Effects Tab/Applied Effects Palette as well as at the Timeline level
The following help page might be helpful:
Hope it helps.
Thanks and regards