When you right click over specific cell of the image in the main window, you will activate context popup menu.

Main image popup menu

First option of this menu is to show original image used as a tile in a new window.
Note: Active only if cell already rendered.

Second option is to erase current tile.
Note: Active only if cell already rendered.

Below is a list of a candidate tiles for replacing the current tile.

When a range of sells is selected you will get another set of options:
Erase selected sells - i.e. erase all cells inside selection.
Render selected sells - i.e. render cells inside selection.
Export Selected to File - this will brings up Export dialogue.