Sony SGPT21 Series Help Manual page 163

You cannot insert/remove a SIM Card.
To insert a SIM card
1. Make sure all the cables are unplugged from your tablet device, and then turn off your tablet device
[Details].
2. Slide off the upper cover.
3. Aline a SIM card with the mark above the slot, and then carefully slide the SIM card into the slot until it clicks
into place.
4. Slide the upper cover back in.
Note
Make sure not to insert the SIM card in the reverse direction.
To remove a SIM card
1. Push in the SIM card all the way and then release it.
The procedures after this step are the same as from step 4 of "To insert a SIM card."