How to make the base color lighter

You can use the dot filter method to add tonal values to the monotone shade. Dry brushing the high spots with a slightly lighter grey will pop details like rivets, bolts, louvers, weld lines etc. Dust pigment shades will also break up the dark tone.

Honestly if you want it lighter, repainting it is the surest bet.

Is the question "what do I use to change existing coat- already dry?"  Or, "what do I had to paint to make it a little lighter shade"?

To make it look like it is chalked from sunlight you can overcoat with a very thin light coat of white or light gray.  But it will tend to be uneven unless you are a real airbrush expert.  On the other hand, sunlight chalking should look a bit uneven, with a heavier coating on top and horizontal surfaces, fading to almost none on vertical ones.  Turn down the paint flow way down, with lower air pressure, to get that thin, transparent coat.  And practice it first on scrap.

My model is already dry! I don’t use an air brush. I would just like a way to make it lighter. I dont really want to repaint it again by hand. This is just my second attempt so I really do not know what I am doing!

Thanks for the replies

German Panzer Grey is very dark as is so the color is correct. I don't understand why you want to make it lighter.

If you still want to lighten it a bit then use some thinner and a lighter gray enamel shade to make a wash and brush it on to act as a filter over the Vallejo acrylic that will produce some tonal changes.