how to change ac frequency
No! Transformers can NOT change frequency.
Mechanical Solution: Use a motor driven by 50Hz source, and the motor drives
a 60Hz generator. I think this will give you a picture of what is needed, and
there are commercial products doing this job.
Electronic Solution: As suggested above, change to DC and change to 60Hz
by an inverter. Again, there are commercial products doing this job, and
depending on your requirements, there are variable frequency models.
S. H.