Have a look at this page http://acer.custhelp.com/app/answers/detail/a_id/2631/session/L3NpZC9XeHljRnd1aw%3D%3D

On some acers when you boot there is an option displayed at the bottom that says
Press Del for BIOS and F12 for setup
If it is like that on your system then press F12

Otherwaise try Alt+F10 at start