Tap/Untap Land and Casting Spells
Basic lands have hidden text that is no longer printed on them. It reads as follows.


  Tap: Add [color] to your mana pool.


This is an activated ability. If you announce that you are activating that ability, and then tap your land, then you have activated that ability. You cannot undo an activated ability.

What if I tap a land, don't announce it, take an action, and then change my mind?

Most people don't announce that they are activating the mana ability on a land. They simply tap it. I had an opponent that tapped all of his land and announced Sphinx&#39;s Revelation for 6. I responded with Skullcrack. My opponent claimed that he had not announced his land activations yet. I called a judge. I personally experienced a Competitive REL floor judge tell that opponent that his intention was obviously to activate the mana abilities, whether or not they were explicitly announced. 

Player intention is often a consideration in judge rulings.

What if I tap a land, don't announce it, take no action, and then change my mind?

You tapped your lands, but you didn't announce it, and you haven't cast anything. I call this "fiddling" with lands. There is nothing in the comprehensive rules, the regular REL rules, or the competitive REL rules that address this. Some unqualified sources on the Internet say you can take it back. Others say you cannot. In general, if the MTG rules do not permit something, it is not permitted.

What I do know is that MTGO allows you to "undo" your land activations, and that top 8 at Grand Prix's and feature matches from StarCityGames tournaments streaming on Twitch.tv have all shown untapping of lands without a judge intervening. In other words, even the top level judges allow untapping lands, as long as you haven't taken any action since you tapped it and nothing triggered when you tapped it.