num0  30
num1  50
num2  70

newval= 20
with open(file, 'r+') as f:
    for line in open(file):
        if line.startswith('num1'):
            val = line.strip().split()[1]
            line = line.replace(str(val),str(new_val))
     f.write(line + "n")
 f.close()