Cheating at dice rolls is when players intentionally alter the result of a roll in order to get a different effect in their game. Surreptitious dice manipulation by the GM may be an expected part of the game's system, in which case it is called fudging instead of cheating, but the same actions by other players is frowned upon.
There are a number of simple ways to alter a dice result, described below.
The easiest way to cheat is to simply lie about the result. Rolling in an obscured location and the disturbing or scooping up the dice will prevent other players from being able to verify the number, but is bad practice whether you are cheating or not and other players may become suspicious.
A way of indirectly influencing a roll result without touching the dice is to make arithmetic errors when calculating or applying modifiers to the roll. This is most likely to remain unnoticed for long and complicated calculations with many hard-to-remember inputs (especially where those inputs only appear on that player's character sheet), but there is always a risk that someone will be able to double-check.
When rolling, do not roll all the dice simultaneously, but pour them out gradually. Aim the falling dice at any low rolls already on the table. If this is done obviously, other players may notice and demand that original result stand or that all dice are rerolled at once.
One method of altering a roll is very simple. As the dice stop rolling, quickly knock or flip over any dice that comes up with a 1. You can wait till others look away, or simply move your hand over rolled dice while your other hand continues to manipulate the remaining dice. The movement is so quick it is virtually impossible to notice out of the corner of one's eye. It requires no precision, since anything is better than a 1.
Like flipping, but keep in mind that is is slower and easier to observe. Remember that most dice are designed such as that the two sides sum up to twice the average roll. For instance, the 3 and 4 are opposite on the d6. Thus, a 1 is opposite the high number.
With a few hours practice, most people can practice "rolling" a die with a simple double or triple flip that reliably lands on the desired number more than half the time. Although rarely practiced, deliberately rolling specific numbers is not terribly difficult, and can be picked up accidentally just from handling dice a lot. The only countermeasures are when someone insists you drop the dice straight down, roll into a cup, use a dice tower, roll against a wall or screen like a craps player, or otherwise disturb the die's original trajectory. In extreme cases if this ability is abused, other players may insist that you never roll your own dice, which could have long-lasting impacts on your fun in future.