Yes you can, plug your iPhone into iTunes, in the left panel right click on your iPhone and create backup. After the backup is complete update your iPhone, when it asks you if you wish to backup your iPhone select know, after the update is complete right click on your iPhone and select restore from backup, select the backup file you just completed.
Yes the backup will contain everything you currently have on your iPhone, my manual backups are always much quicker then creating a backup before an update. If you are still weary and would like to trouble shooting the backup issue here are the steps.
thank you, ill just ignore the message then. Ill keep you updated !
edit: I found this solution, which I think is more save. Going to try it.
1. Create backup your iPhone.
2. Save the backup folder (eg.: another drive)
3. Click RESTORE button in iTunes -> Your iPhone will updated to iOS5, and will be empty (like when you bought)
4. After restore iTunes ask : Set up as new device or Restore from Backup...
5. If you select Restore from Backup your device will be the same (some minutes later) like before you start the process, except your iPhone has a new iOS5.