Hello Alberto Diaz,
firstly you have to check wich is de boot's ID you want to replace. I think you can do this with CG File Explorer 16 v 0.7 (you can find this tool in the editing software forum). Once you know that ID you must rename the .rx3 you have downloaded with the generic boot ID and paste that renamed file in the correct directory. For example:
generic boot ID: shoe_904_0_textures.rx3
.rx3 ID donwloaded: shoe_200_0_textures.rx3
NEW .rx3 ID donwloaded (renamed): shoe_904_0_textures.rx3
I hope this can be useful for you