“Mr” (without a dot) is standard in the UK and “Mr.” (with a period) is standard in the USA (ref: Longman Guide to English Usage 1988). In the interests of international relations you can toggle between the two by using the 
+period (in the Main window hold down the key and press the '.' (period) key). If you don’t like the results you can undo them.
If you want to use “Mr.” (with a period) as standard you should edit the titles available in the Titles tab of the 
Preferences window.
See also: