[NASA] has been adjusting temperatures from the past[...] downward, while adjusting current-day temperatures upward, and those changes are responsible for most of the claimed global warming during that time.
Scientists at NASA—as well as other groups—constantly work to ensure that the data being used to estimate global average temperatures are as accurate as possible. As time goes on, updates can lead to small changes to estimates for previous years. These changes, however, are much too small to cause the warming trend that is clear in all available datasets.