Hi Avoid drinking this if the taste changes..

Usually if the coconut water tastes acidic it means it is fermenting,or may be there is some microbial growth. Usually fresh coconut water is alkaline and has the electrolytes like sodium,potassium,salts .So alkaline taste. If is fresh and tastes acidic means sometimes the soil in which it is grown may be has such qualities which is imbibed by the coconut water.Make sure that there is no breakage in the coconut  shell for the microbes to enter or the coconut water is stale.