You can edit a quality in the Quality module: under Definitions (section SCHEDULING), open Quality related, then Quality. To edit, select a quality and click Edit 
.
You can change the values that you have set:
If you select a machine quality that is already used in the application, you can also change these values, but you need to unlock the quality first:
If you click the Unlock button, you will get a list that indicates all places where the machine quality is used. Sometimes it is better to create a new quality if the quality is used in a lot of places.
If you then click Unlock, you can make the changes to the quality. A new version of the quality will be created.