generating ultrasonic sound in matlab
To answer the "how to shift the frequency of an audio signal up" bit:


You could multiply the signal by a sine wave at a high frequency. This would shift and mirror the whole spectrum of the original signal into the high frequencies (multiplication by a sine in the time domain = convolution by a pair of symmetric Dirac in the frequency domain) - the mirror image could then be removed by high-pass filtering. Limitation: this would not preserve integer ratios between harmonics. For example, a harmonic mixture with partials at 100, 200 and 300 Hz would have partials at 1100, 1200 and 1300 Hz if shifted by 1kHz - and the integer ratio between harmonics is lost, making the resulting audio signal "metallic" or "dissonant" - at least to human hearing's perception criteria!.
You could simply play back the original data at a higher sample rate. Limitation: multiplying the sample rate will also reduce the duration of the original data.
Actual musical pitch-shifting is done with techniques such as the phase vocoder (Short-Term Fourier Transform).


However, you probably don't need to bother with any of these methods because:


There is no need to shift an existing audio signal - given that the content of the original signal is of little relevance and will be degraded in one way or the other by the transposition process, why not directly generating a tone at the required frequency to start with?
Keep in mind that whatever you do, the converters and transducers that come with a computer cannot reproduce ultrasounds (they have a cutoff at the upper limit of the human hearing range, about 20kHz).
The evidence that ultrasounds have a repellent effect on mosquitoes is very shallow.