Well, the camera adjusts exposure up & down automatically with changing light. There's no manual mode - or exposure lock - like on advanced DSLRs. Even GoPro doesn't have exposure lock (there's a GoPro owner petition to get them to add this).
Best to lock down everything you can, to minimize sudden exposure changes:
1) Set ISO to 100 (not AUTO)
2) Set White Balance to Sunny (not AUTO)
3) Set Exposure Metering to Average (empty circle icon, no dot). The camera will use most of the image when evaluating exposure, rather than just a tiny part of the center - which could change rapidly.
EDIT: at least I *think* the empty circle is Average metering. Hard to tell from the manual the way the icons are laid out. For sure the single dot is Spot metering. The dot within a circle icon could be Center-Weighted (like on Nikon), or it could be Evaluative (aka whole-scene analysis, aka Average) like on Canon! Hmmmm...