Well, tutorials exist that cover texturing. Re-texturing is basically just the same, other than you are choosing to replace an already existing texture file.
Two choices for that replacing bit.
1) create/rename a texture(s) to the same name(s) as the texture used in the one you wish to change. IE, if your palceable currently uses a texture named plc_mytext you could create a NEW texture of the same name, and place it in a hak or your override folder.
2) edit the placeable mdl file and choose a NEW name for the textures used, and edit as necessary any UVW issues you may find/have. Then export the mdl again. Likely best choice would be to rename it as well.
There is a list of many tutorials found
here.
Anything to do with texturing will be applicable to your quest. This includes UVW mapping, or general texturing to creating/editing a texture in photoshop/gimp or texture editor of your choice.