why is the volume up button not working to pass the yellow triangle warning on my s3 i9300?

[AC Question]

i wanna update the 4.0.4 to 4.3 or latest ones , but even if i do have resources on my pc the problem is i think its on my device GT-I9300 , when i get to the download mode the warning mesage comes, saying A custom OS can cause critical problems in phone and installed apps.
volume up: continue
volume down: cancel (restart phone)
here , i wanna continue to perform the update system but the up key doesnt let me to go, when i press nothing happen, but the down one works as it says restarts the phone, ANY ONE HELP HOW CAN I PASS THIS PROBLEM OR SOLVE IT ? thank you !!

 

[Tim1954]

Hmmm, it's usually the power button that allows stuff in Samsung devices... I hope you backed all your files up....

 

[B. Diddy]

Welcome to Android Central! Maybe your Vol Up button is broken. During normal operation of the phone, does it actually increase your volume when you press it?