As kingtobbe says, if you just want to replace the texture on the sign, you can just drop the texture file into your data folder in the same location in the file tree as the original would be. Make sure you dive the texture the same name as the original so that the game will know what to do with it. If you want to make a standalone version of the sign that has your texture or something like that, you will need to go about things differently. I won't go into detail on that if you just want to do a straight texture swap, but do let us know if you are wanting to do something fancy that needs a material swap. I usually use Creation Kit, so can walk you through that if need be.