Can't restore SQL 2008 backup - error that the database is in use but it isn't
It's only related to being your default database, because this is the db context when you connect. This is not the actual issue.
All you have to do is put "use master" above the restore to change the database context.
It may be that one of your clients relies on there being a default database... so changing it may cause issues later on.
You can't restore the database when it is your default database. Check your login, and see what is set as the default database. Try setting the default database to 'master' and retry the restore.