I have an Acer Icon A3 and need to change sleep time

[Hathor43]

I need to change the time of sleep mode to longer than 30 seconds and also to remove the demo mode in the left lower corner. have an Acer Iconia A3 and have disabled demo mode but demo mode is in blue down in left hand corner of screen. I cant get the sleep mode changed as it shuts off in 30 seconds. I went to settings and the only choice in sleep is 30 seconds. How do I fix this. Any clues?