You can manipulate the contents of memory using the Poke Tool, but the interface for this is severely limited by space constraints: For more than the simplest editing, you will probably find the integrated hex editor far more convenient.
Nonetheless, to view and edit values within the circuit, the Poke Tool has two modes of operation: You can edit the address displayed, and you can edit an individual value.
To edit the address displayed, click outside the display rectangle. Logisim will draw a red rectangle around the top address.
To edit a particular value, click the value within the display rectangle. Logisim will draw a red rectangle around that address.
Next: Pop-up menus and files.