Drinking the daily recommended amount of water (64 fl. oz. a day) in conjunction with a healthy diet and exercise will speed up your metabolism - your body will burn through calories, nutrients, and THC faster.
THC does not "burn" in the body. THC is stored in the fat cells. When fat cells burn, the THC is metabolized into THCA and released back into the bloodstream. From there, the kidneys filter the THCA from the bloodstream and deposits them into the bladder. A sizeable portion of THCA is excreted via the feces, but since feces is not used as a specimen for drug tests, urine is what you should be concerned with.

Also, drinking water days or weeks before the U/A is of no help. It may cause frequent urination, but the rate in which the kidneys filter the THCA from the bloodstream remains unaffected.