Instructions:

1.8.1 file:
Minecraft 1.8 Jar

For Windows

1. Set Minecraft to defult texturepack to avoide any problems.
2. Close Minecraft
3. Go to start and in search box type in "%appdata%"
4. This should bring you to C:\Users\[yourusername]\AppData\Roaming
5. Rename minecraft.jar in the bin folder to somthing else so you have a backup.
6. Copy over the jar file from the download above.
7. Rename it to minecraft.jar

For Mac

1. Set Minecraft to default texturepack to avoide any problems.
2. Close Minecraft
3. Browse to your user directory.
4. Select 'Library', then 'Application Support'
5. Rename minecraft.jar in the bin folder to somthing else so you have a backup.
6. Copy over the jar file from the download above.
7. Rename it to minecraft.jar