i mean what's the next stwp

Assuming you have a compatible device, see: Update your iPhone, iPad, or iPod touch - Apple Support

And: If your iPhone, iPad, or iPod touch won't update - Apple Support

If the above Apple articles do not help, please post back with relevant information. 

If your device is running iOS 12.5.5, it can be updated to iOS 12.5.6, no further.