Q:

My ~ and ± keys are switched, how can I switch back? 

As the title says the keys described above have been switched, how can I switch them on Mac OS X? 

A:

Have a look at this program.  It should sort your problem out in no time.  http://pqrs.org/macosx/keyremap4macbook/
However this one works as well: 
http://scripts.sil.org/cms/scripts/page.php?site_id=nrsi&id=ukelele
Take your pick!