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 video used as a tile clip in a new window.
Note: Active only if the cell is already rendered.

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

Below is a list of a candidate videos for replacing the current cell.

You may change the second option if you press Alt key. It will be changed to:
Remove This Image from Mosaic and Library...

This will removes all occurrences of the given video from the mosaic cell and also remove it from the library.

Please note that all videos or images in the library will be renumbered. If you will want to fill resulted gaps with respect to Distance setting you should re-render the entire mosaic.

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.