First you should change the default frequency in the simulation options to 50 Hz. Then please edit all the data forms (windmill, transformer, load, sources, machines, etc) and change to 50 Hz. You must also go into the DFIG subcircuit and change the converter transformer frequency manually.
Also please note that some filters and dq0 transformations used in the DFIG controls are tuned around 60 Hz so you should change the transfer functions and the base frequency accordingly.