how to backup iphone if touchscreen is not working
I tried to backup my phone on itunes but because i cant press 'trust computer' on my phone i cant backup my phone.
You will have to get it repaired or replaced. The touch screen interface is the only way to input commands to an iPhone so until you get a working screen you cannot backup.. The whole point of backups is to make them regularly so they are already in place when a problem arises. Once a problem comes up, it's usually too late to make a backup.
DId you by any chance use iCloud backups? Those are automatic, occuring daiky when the phine is plugged in, connected to a wifi network, and in sleep mode.