You want to reset phone's memory data or internal memory ?
For phone's memory
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Backup everything as it will erase all data on phone (sms, contacts,apps)
Option 1 ) settings > backup and reset > factory data reset
Option 2 ) Reinstall the phone's software SUS ( Sony Update Service) ... download and install it on pc and follow the on-screen instructions
http://www.sonymobile.com/global-en/tools/update-service/
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For internal memory this
settings > storage > clear internal storage
