| Clean the frets first. I use Brasso and a soft cotton cloth myself because it works and I always have some around. I mask off the fingerboard to avoid getting it stained. It's a quick job.
If the frets are badly worn you may need to get them levelled and dressed if there is enough fret left to do this. If not, then you can consider replacing the frets.
If after you've cleaned them and the wear looks excessive, take it into the Twelfth Fret on Danforth at Woodbine. They do good work, but they're not cheap.
Sometimes it can be cheaper to replace the neck or the bass. | 
