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I have been told to check the sim card. How do I remove the back
You don't need to remove the back cover to check or replace the SIM card:
1. Open the nano SIM/Memory card slot cover.
2. Using your fingernail, drag out the nano SIM card holder.
3. Remove the nano SIM card, then re-insert the holder.
4. Insert the holder into the slot, then close the cover.
More info here:
https://support.sonymobile.com/global-en/xperiae5/userguide/assembly/