Work permit extension job description place of work changing

I've worked in Thailand for almost a year now and here I am extending my work permit for 1 more year. Can I just simply change my job in it and place of work ( to the other branch of the company) anytime I decide or I have to apply for a new one? And how much does it usually cost if I get all my papers myself ready and come to apply for it in the labor department for farang (silly me - I still have no clue how does it called ) Can someone advise a thai helper for that in Hua Hin area a reasonable price?