how to change ISO while seeing from viewfinder in my camera canon 70d?
Press the button labeled 'ISO' once, turn the 'Main Dial' or the 'Quick Control Dial' until you have the set ISO to the value you wish, then press the 'ISO' button again or half-press the shutter button.
The 'ISO' button is on the top left of the camera just in front of the top LCD and just behind the main dial.
The 'Main Dial' is right behind the shutter button.
The 'Quick Control Dial' is the large wheel on the lower right of the camera's back, surrounding the 'Set' button and 'Multi-controller'.
If you see an 'L' in the viewfinder when you try to change the ISO, it means you have set the 'Lock' lever on the lower right of the back of the camera and attached the 'Main Dial' or 'Quick Control Dial' to the controls locked when the lever is in the up position. Flip the lever down to so that the exposure values attached to it are unlocked.