how can i delete only some record cases while the questionar is still online ?
Why would you like to do that?
Deleting data during data collection is quite hazardous, therefore SoSci Survey will not allow for that. Too many people have lost data by trying that and doing something wrong.
If you are really sure, you can go offline, delete the records, and then go online, again.
But it is much more intelligent to simply exclude the respective cases from analyses and download. Please go into the menu option that allows for setting the selection criteria for valid cases. There enter which cases you would not like to use.