Shut down the computer and insert the USB-stick.
Then hold the F2 key before hitting the power button and don't release it until you get to the bios settings. You may hear a loud beep, don't be alarmed, that's normal. Once you're inside the bios, you will have to change the boot order. Use the arrow keys and the return key for navigation. The exact wording may vary, but set it up, so that the pc checks for a bootable usb stick first and then for the drive you want to install Ubuntu on.
Then look for an option like "save and exit". Your pc will then reboot to the Ubuntu USB-stick. The installation process from the stick is self-explanatory.
After the installation, you may want to revisit the bios settings and disable all boot options except the drive you just installed Ubuntu on, to speed up the booting process in the future.
Have fun with your device!
Edit: The F2 key works on most acer machines. I myself have an Acer Aspire ES15 and I installed it that way. If it does not work, try all the other function keys, the DEL key, and the Esc key. If all of that does not work, your machine must be haunted.