You are gonna need atleast 2 things...
Unity Asset Explorer (UAE) And some kind of photoshop (i'd recommend Paint.net)

Links:
Unity Asset Explorer (UAE):
http://zenhax.com/viewtopic.php?t=36
(anton gave me this link, its safe)
Paint.net:
https://www.getpaint.net/

They are both completley free!

Once you have downloaded and installed UAE, go find the exe and open it up. There should be a folder with the exe in it, under your downloads.

Heres what the software looks like:

(Sorry if it may be low qaulity, the software uses a old font)

Now go to the top left corner, where it says "Open Asset-File"
It will bring up a file explorer and your gonna want to make your way to the
"AmazingFrog_Data" folder.
It will give you several different folders, and 21 Assets

Now you can choose one of these .assets, and click "open" in the bottom right corner of that menu.

Most things are in resources.assets, but there are tons of things in 1-21 sharedassets


Once you select a file, right click it and hit "extract". Now go back to the downloaded folder with the UAE exe. In it should be another folder, titled "resource.assets" or "sharedassets1" or whatever .asset you opened. In there you will find the file. If its an image, you can view it and if its audio, you can listen to it.

Thats basicly how to get all of amazing frog's files

Once you have found the texture you want to change ( a costume, car, wall, etc) open it up in your choice of photoshop. Now edit the texture to how you please.. and save it. Make sure that the edited texture is the same size as the original (in mbs).

You can ensure this by:
changing Bit Depth,
generating bitmaps
and making sure its the correct one of these: DXT1, DXT3, or DXT5

Once you have saved your image, go back to the UAE menu and find the exact file that you extracted and edited. Now right click it, and hit "Import DDS".
If it doesnt work, you may have the wrong file size. Keep trying. (if you get anything with a "ERROR!" then you know you did it wrong)

Once you have successfully imported the dds, go to the top right corner of the menu and hit "save asset file" now lets say you were in resources.assets. Your going to want to click on resources.assets and overwrite it. If you were in sharedassets4, overwrite sharedassets4. This goes for every asset.

Warning: this is what changes the game, or modifys it. If you break something, reinstall the game

So once the asset file is replaced with the editied one, (restart the game if it was open) open the game and check it out!

.tex are usually images/textures
.ogg or .mp3 are audio files
.mat are presumably models

Alright,
Step 1
Navigate yourself to "sharedassets2" where you will find a file "iLoveCats_Diffuse"
Now do the good old Now go fetch that file
Step 2
Now put the extracted image (it should say "i love cats". however beagles are better) into your means of photoshop, or paint.net. Now you can edit this image, or paste an image over it.

IMPORTANT: You CANNOT make the file any bigger or smaller. If you change the size it probaly wont work (thats what she said?). Keep it at 256x256 pixels.

It also needs to be the same file size (roughly 86.5 mbs). You may need to mess with bitmaps or other renderings to make the file size match 86.5 mbs

Step 3
Once you have the new shirt logo and it's the correct size, repeat the process in part 3.
Get it imported, save the asset file and then open/relaunch you're game.