There is an unindent
command that can do this for you, either a line at a time or as multiple lines at once. You can find the command in the menu under Edit > Line > Unindent
or use the keyboard shortcut that you see there (Ctrl+[ on Windows/Linux or ⌘+[ on MacOS by default).
If there’s no selection, the command will unindent the current line, whereas when there is a selection it will unindent all of the lines contained in the selection (even if they’re not fully selected).
The inverse of this command is the indent
command, which uses the ] key in the bindings above instead and will indent the current line or selected lines.