I am assuming you want to clean it while full. Can't help with that. If it is not occupied, I have done this successfully several times. You will need silicone and a razor blade at the bare minimum. You have to remove the old silicone then use the blade to carefully but thoroughly remove the crud and silicone remnants. Sometimes crud is actually etched into the glass that's when you will need a powder polish like cerium oxide, which you can also use to buff scratches out of a tank. Just make sure you rinse it good and get rid of all the powder before you reseal the tank. Use the buffer wet, nice and wet. If its not etched in, but its hardened like calcium carbonate that is common from saltwater, then EDTA and a toothbrush works great and wont scratch the glass. you can also use this method to get that impossible crud offf of glass tops.
Silicone is best applied in a small bead running down each side which is smoothed by a gloved finger. Look for any bubles or missed spots. it usually leaves a smoother finish if you wait till the first coat has hardened most the way before filling in gaps. If you have more specific questions pm me. I pretty much only buy used tanks, I fix them up before stocking them with fish / shrimp.