the same way you clip out any other time. REEEEELLLAAAAXXX. You're probably getting flusterd a little on the climb thinking "ohGODI'mclimbingandIdon'tthinkIamgonnaunclipintime..." when you should be concentrating on the trail and twisting your heel out like any other time you want to unclip.
If you're standing you might be pulling up particularly hard on the pedal, which makes it a little harder to unclip sometimes. Let off on the pressure a little. To practice this, get on a flat stretch and shift into a too-hard gear. Pedal hard in this ratio and practice clipping out at the 12/3/6/9 o'clock positions with one foot at a time.