Mp3 Audio Editor can convert a file's sample rate or number of channels. This change applies to the whole file (you cannot change just parts of the file).
To convert the sample rate of the current file use the menu File -> Convert Sample Rate and Channels and select the new sample rate and channel. The sample rate must be between 6000 and 96000 samples per second. Typical sample rates are displayed in the pull down list.
A typical reason for converting the sample rate down is when you know it is going to be used on the telephone (8000 mono) or if you are about to compress the file for internet transmission (eg. GSM takes 11025 or 8000 mono). For a broad explanation about sample rates see General Audio Concepts.