Select Outdent from the Block menu to outdent or unindent each line in the selected portion of the current file. EditPad does this by deleting a certain number of spaces from the start of each line. How many spaces are inserted depends on the "block indent" setting for the type of file you are editing. You can change this setting on the Editor page in the file type configuration. If there are not that many spaces at the beginning of a certain line, all the spaces at the beginning of that line will be deleted. But the other lines will still be outdented by the specified amount.
If the selection spans more than one line, you can also outdent it by pressing Shift+Tab on the keyboard. The difference between using Block|Outdent and the Tab key is that Block|Outdent uses the "block indent" setting while Tab uses the "tab size" setting to determine the amount.
If a rectangular block is selected, Block|Outdent works differently. Instead of deleting spaces or tabs at the start of the line, it will delete spaces and tabs immediately to the left of the first column in the selection, effectively shifting the block a number of spaces to the left. Spaces inside the selected block will not be deleted.
Block|Move Lines and Columns