SetOptions

SetOptions[s, name₁->value₁, name₂->value₂, ...]
sets the specified default options for a symbol s.

SetOptions[stream, ...] or SetOptions["name", ...]
sets options associated with a particular stream.

SetOptions[object, ...]
sets options associated with an external object such as a NotebookObject.

SetOptions is equivalent to an assignment which redefines certain elements of the list Options[s] of default options.

You can use SetOptions on InputStream and OutputStream objects. If there is only one stream with a particular name, you can give the name as a string as the argument of Options.

If you use SetOptions[NotebookObject[...], ...] the kernel will send a request to the front end which will immediately make the change specified.

Reset the default precision for HilbertMatrix:

Now HilbertMatrix uses machine numbers by default:

Change the style for plots to use dashed curves: