How to forget a profession

To unlearn a profession, you should find and use the appropriate Job Deleveling potion, crafted by an Alchemist lv 1, and usually made with 2x Potion of Old Age and the tool of the profession you want to forget. My advice is to collect the resources, really cheap, and find an alchemist who can help you crafting it. The second option, otherwise, is to buy the potion from the market.

If you were in Incarnam, you could unlearn some profession from Giles Caper, in the Incarnam Inn (only Fisherman, Farmer, Alchemist, or Lumberjack - the same four you can learn from Foreman Ikure in Incarnam).