Acer DX900 User Guide - Page 228

12-4 If you checked "Auto backup on schedule," tap to enter the screen shown on the left. Backup frequency. Tap the date button on the right of "Starting" to set the starting date. Tap the time button on the right of "Starting" to set the starting time. If you have selected "Weekly," tap the function button on the right of "Every" to set the day of the week to backup. If you selected "Manage old backup files," tap to enter the screen shown on the left. Tap the arrowhead to set the number of files. Tap the files to be retained.