This guide is not exactly for your model, but it will probably work on your model, too:

Turn on safe mode

1. Off.
2. Press and hold to On.
3. When "Acer "logo is displayed, release the key On.
4. Immediately after releasing the Power key, hold the key Volume down.
5. Keep holding down the Volume Down key until the phone finishes rebooting.
6. Release the key Volume down when you see Safe Mode appear in the lower left corner of the screen.

Source: http://howtosafemode.com/843/how-to-turn-on-safe-mode-on-acer-iconia-tab-7-a1-713?show=845#a845

This guide is not exactly for your model, but it will probably work on your model, too:

Turn on safe mode

1. Turn device off.
2. Press power button.
3. When the "Acer" logo is displayed, release the power button.
4. Immediately after releasing the power buton, hold the volume down button.
5. Keep holding down the volume down button until the phone finishes rebooting.
6. Release the volume down button when you see Safe Mode appear in the lower left corner of the screen.

Source: http://howtosafemode.com/843/how-to-turn-on-safe-mode-on-acer-iconia-tab-7-a1-713?show=845#a845