Will OS upgrade from Snow Leopard to Mavericks erase other partitions? I have Macbook6,1 Late 2009 Unibody white Snow Leopard with dual boot Debian Wheezy installed on the other partition. 
I use EFI to show boot options at startup. 

Will upgrading to Mavericks overwrite/delete other partitions?

Debian is what I use primarily. I would however like to upgrade to Mavericks for security reasons.
Updating will not affect any partitions you do not choose in the set-up process. If you're not asked, then it will install on the partition the OS X install originated on.