MEP Plus, but which exact version? Please check under Help>About.
The most obvious thing to try is adjusting the colour and brightness/contrast controls, but in MEP these are quite limited in scope. VPX gives you rather more control and adjustment.
Your problem really will be, how to affect the sky without also adversely affecting the whole scene, or is it a case you really want to lighten everything? Whichever, creating a "blue" sky on an otherwise dull day is challenging!
I don't know of any plug-in that would allow you to do this; I'm sure other users will join in if they have ideas to offer.