From a user perspective, you can change ratings from various places on the application.
As manager you have more options:
Go to Profile Page. You will find all skills you have assessed on this page. This is where you can change each value to keep it up to date.
On a skill dialog, you can change your self-assessment values. This is where you can also set the level you would like to achieve in the near future, setting the desired type of mark.
Your manager can send you recommendations of skills you should learn and improve in with the levels you should try to reach. You will be able to see those skills on your Career Page. You can also change your self-assessment values while exploring career opportunities.
Explore the map to see which of your skills and skill levels have been included. If you find a skill on the map which is set at an out of date or incorrect level, don’t hesitate to update it to reflect your current skills.