Please solve the programming task below using a self-contained code snippet in a markdown code block.

You are given a positive integer num consisting only of digits 6 and 9.
Return the maximum number you can get by changing at most one digit (6 becomes 9, and 9 becomes 6).
 
Please complete the following python code precisely:
```python
class Solution:
    def maximum69Number (self, num: int) -> int:
```