I am not sure there is a way to open up the G&P aimpoint... how did you get dirt inside the tube?
According to G&P it should be "High shockproof, weather proof & Fog Proof model, functions in bad climate and different situation" However if some orings around the windage/elevation adjustment knobs went missing I could potentially see condensation getting into it... but not dirt... that would take submersion into mud water I would think...
Anyway, you might be boned on this... mabey not, but looking at mine it seems that when G&P made these things, the idea was that they won't come apart no matter what you do.
Replacement = 34.98USD
http://cgi.ebay.ca/G-P-30mm-Aimpoint...QQcmdZViewItem