There are two ways to upgrade to 13.04 without loosing your data:

• You can upgrade your current install

You will get a promt asking you to upgrade in a couple of days (2 weeks after release I think), or you can upgrade now by typing sudo do-release-upgrade in a terminal

• You can install over your current installation without formating the drive

  
  
  
  
  
  • Download ubuntu 13.04 and create a startup USB disk or burn it to a CD/DVD.
  
  
  • Install as you normally would.
  
  
  • When asked select "Upgrade Ubuntu 12.10 to 13.04"
  
  

Your data should not be overwritten, but it is good practice to back up your data anyway.

If you have a more complicated set up (such as separate /home partition) you can select "something else" and assing your partitions as you did before. But make sure not to put the checkmark to format you "/home" partition