| Mounting the PSU upside down is safe and will not cause issues. Any dust in your case is going to where the most airflow is anyway so the position inside the case for the PSU is not critical with regards to dust. If you find the dust in your case is an issue, normal cleaing with compressed air is a good idea.
Buying another PSU is not going to solve the issue. Only a new case would change the mounting orientation.
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