Go to yourName-->setup-->Data management-->storage usage Check which objects store the maximum storage, then find the unwanted records in that object and then delete those records.
It look like you already used your 5MB space in your developer org. Please follow below step 1) Go to yourName-->setup-->Data management-->storage usage 2) Delete unwanted data
Check which objects store the maximum storage, then find the unwanted records in that object and then delete those records.
Please follow below step
1) Go to yourName-->setup-->Data management-->storage usage
2) Delete unwanted data
Let us k now if this will help you