Here's a nother fun and easy way of replaceing the doom1.wad by using xarchiver, x-terminal and, emelfm2. (this is for if you are not good at using the root shell or if you are having problems)
You aswell need at least 19mb of system memory free inorder to do this...

1. download the doom2.wad and unpack it using xarchiver and extract it into mmc1 or mmc2
2. using your file file manager, rename the wad file to doom1.wad
3. open x-terminal and enter "sudio gainroot" and then after you've enabled the root shell enter "emelfm2" (this will cause emelfm2 to go into root mode so you can replace the exzisting wad file)
4. split the screen so you can see two directorys at once, you can do this by pressing the button that is last in the upper left hand corner. (if you allrety have a split screen or you know how to do this skip this step)
5. on one side go to the directory /usr/share/lxdoom and on the other side go to /media/mmc1 or /media/mmc2 depending on ware you have the wad file
6. using your sylis, drag the wad file frome /media/mmc1 or 2 and drop it into the /usr/share/lxdoom, it will ask you if you want to override the current file, tap yes (it may take a while because the new wad file is 17mb in size)
7. after it is done, close all applications, now you've successfuly updated lxdoom!

Tell me how it went
Enjoy!!!!