How do you fix bad sectors on hard drive WD i HTS54 External

The first thing you need to do is to copy all irreplaceable data to another drive or optical disks, if you haven't done so already. Bad sectors don't get fixed. They get marked as bad so they no longer get written to. A few bad sectors isn't much of an issue, but if they continue to increase, the drive will need to be replaced. Run WD's diagnostic software. If you don't understand the results, post them here and someone can explain them to you.

Is it an old hard drive? Does it still work well?
You should always learn to back up your important data on a separate drive from now on in case of any data loss.

I have the same issue with an internal hard drive, I think the normal count for bad sectors would be around 4-6 bad sectors, more than that means the hard drive is going to die soon.
What program gave you the count? I recommend using spinrite or HD regenerator.
Good Luck.