如果我想返舊公司做，應該點做？？

If you do not like the new company, then you should decide asap. But if you want to go back to the old company, you should ask yourself some question.
1) When you leave the old company, is it peacefully quit?
2) Whether the old company allows me to enter? Your position is being occupied by someone?
3) The salary and benefit may be changed if you go back to the old company.
If you think it is not a question, then you should approach your boss and discuss it asap.