You can find the schema by the name in the configuration (System Designer -> Advanced settings -> Configuration). Please note that there may be a few schemas with the same name belonging to different packages. The schemas with the user customization are placed in the Custom package and if there is no schema in that package you'll need to create it to be able to apply any changes.
Sorry, I mean by script. Not on the UI.
Something like .EntitySchemaManager.GetInstanceByName("SysSchema") in C#