I want to change the default storage memory so when i download any app from play store it will be installed automatically on the sd card .. my mobile is s7 edge (android nougat)
It does not work that way. App installs always go to the internal storage.
Some apps let some of their parts be moved to the SD card, but it is not worth the hassle.
Your phone may or may not let you format the SD card as an extension of the internal storage. This has pros and cons.
The SD card is best used for your photos, music, videos, and documents.