You can reset a user's skill by inactivity/ lack of use or by a retrain date.
Resetting a user's skill occurs when the user has not used that skill in the set timeframe.
Resetting a user's skill by a retrain date simply means that the person needs to be retrained every X amount of days even though they've used it everyday.
Skill Inactivity Reset by Site
You can reset a user's skill to the lowest level by going to Skills > Settings > enter in a number of days. This setting will affect all Skills on the Site.
Skill Inactivity Reset by Skill
Alternatively, you can reset a user's skill to the lowest level by individual skill by opening a Skill, clicking Edit, and entering in a number of days in the 'Reset Days' box. If this field has a value, it will ignore the Site Setting default value. If this field does not have a value, it will use the default value in the Site Settings.
User Retrain Date
You can also reset a user's skill level by a retrain period. This allows you to choose the number of days before the user needs to be trained again.