The Corona Labs Site has put up a tutorial I wrote on the basics of updating mobile content at run-time:

Updating Mobile Game Content in Runtime

Here is a brief summary:

We take the green tank tile sheet. And use it to place and animation on the screen:

Next we use ad XML file and the network.download() function in Corona to update the tile sheet to a new version when content has been updated:

<data>
<lastupdate>20130401</lastupdate>
<tilesheet>blue_tanks.png</tilesheet>
</data>

This updates the tile sheet in the app:

All of the detail, code, and example files are on the Corona Labs Blog