You can select a block of text using mouse. Just press (and hold) the left mouse button to define the beginning of the block and drag mouse pointer to move the end of the block. Release left mouse button to define the end of the block. While dragging you can move the mouse pointer near the edge of the view for scrolling the view. You can also scroll the view using keyboard while dragging: try Home, End, Page Down, and Page Up, and also arrows, Ctrl+Left, and Ctrl+Right keys.
Click the left mouse button to cancel the selected block. Double-click a word to select the whole word (a word contains alphanumeric characters and underscore (_) characters). Press (and hold) the Ctrl key and double-click a line to select the whole line.
You can extend the selected block by mouse or keyboard. Just press (and hold) Shift key and click the left mouse button to define the new end of the block. You can also drag mouse pointer to move the end of the block. Or press (and hold) Shift key and use arrows to move the end of the block in Text mode. Moreover you can use Home, End, Ctrl+Home, and Ctrl+End keys while holding Shift key to move the end of the block to begin or end of the line in Text mode or begin or end of the file in both Text and Hex modes.
If you need to select all text in the file currently opened in the viewer, use the Select All (the Ctrl+A key) command from the Edit menu or right-click context menu in the viewer.