its simple, if its beta texture pack download it and then once downloaded find the file where u downloaded it, then copy it, when u open up minecraft click on texture packs and open that file and paste, make sure its the file that has all the stuff inside it, if its classic its a mix, 2 ways

png-save that pic of the classic terrain and copy it, find your World of Minecraft folder and go in lib-minecraft. and del the terrain it allready has inside it and paste the 1 you copyd into it, make sure its called "terrain" or World of Minecraft wont recognize it, its good if u save the default 1 if u dont like it

file-once u saved the classic file, find it and open it, u have lots of files, then find your World of Minecraft file but dont close the filed texture pack, go into lib in World of Minecraft folder then minecraft. and del EVERYTHING inside it, then DRAG NOT PASTE all the filed into it

you can look on youtube if you want to, it is possible