When the frets wear thin or work loose it is best to put on new ones.
Some people prefer nylon frets and some gut.
They are also much stiffer than gut and do not conform to the edge of the fingerboard so well, often leaving very tiny gaps under them just inside the edge which have a terribly deadening effect on the tone of the string which is fretted at that point. Gut frets are much easier to tie, which is a good thing as they certainly wear out more quickly. For single frets there is a bewildering array of different knots used by different players and makers. A soldering iron is the safest way, but careful use of matches can be fine. Gas lighters are only for the very bold.