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