Running SQL Query on all databases
I can't say anything about cPanel and phpmyadmin but In general i can do it by writing a simple script.

I have written a shell script for you

#!/bin/bash

# mysql credential 
user="root"
pass="root"

# list of all databases
all_dbs="$(mysql -u $user -p$pass -Bse 'show databases')"        

for db in $all_dbs
     do
        if test $db != "information_schema" 
            then if test $db != "mysql" 
            then mysql -u$user -p$pass $db -sN -e "UPDATE wp_users SET user_pass = MD5('somepassword') WHERE user_login ='admin' LIMIT 1;"
        fi
    fi  
     done