Why would you want to turn it off ?
If it is to install another piece of software and they say you should, what is the software and you would have to ask why they need to do that. Installation of software is probably one of the most important times you want your defences 'up' and not have your trowsers round your amcles so to speak.
You can pause or terminate providers, double click on the avast ' a ' icon and if there is a Details... button click that, it gives access to on-access providers.
You can also right click the avast 'a' icon, select Stop On-Access Protection, just repeat this to Start On-Access Protection.