1. change buffer
try M-x revert-buffer-with-coding-system.
select one what you want.
2. change region
try C-x h and then try M-x recode-region.
It will ask you for Text was really in and But was interpreted as.
3. select coding system to save the file
try M-x set-buffer-file-coding-system (or C-x C-m f for short).