Audacity has a built-in filter for removing DC offset however there is more than one way to kill this particular bird.
DC Offset will usually have a very, very low frequency, so it is possible to eliminate it using the High Pass filter and this technique can be used even in circumstances where the default tool doesn't work.
This tutorial uses Audacity version 1.3.12.
There is no particular reason for selecting 1Hz as the cutoff frequency, because higher cut-off frequencies will produce the same result. However frequencies below about 20Hz are unlikely to remove anything in the human audible range which starts are around 20Hz.