If you mean how do you go back to the original texture then just go to the Options... menu, then select Resource Packs..., and select Default before you enter a game. You can go back and forth without any problems.
Don't forget it has to be de-compressed by Minecraftwhen you select the texture pack so depending on if you have a "realistic" type HD texture it could take quite a few seconds to go from the Default texture pack to the new one.
If you want to get rid of the newer one then go to the folder you put the new one in and remove the texture pack to the trash.
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It will re install your minecraft back to its original state.
if you log in to minecraft again it will resave minecraft follders you are missing!
Shift
go back in time
no