Using the paste_replace_word command, you can paste the current clipboard on the current word without having to make a selection first. For example, copy some text to the clipboard, then move the cursor to the word you want to replace. Open the SlickEdit command line (press Esc or click in the message area) and type paste_replace_word, then press Enter.
If you plan to use the command frequently, bind it to a key or key sequence (Tools > Options > Key Bindings). See "key bindings" in the Help > Index for more information.