HI Reenu,
That is the issue. Once the product is configured to be managed as collective processing and created/posted transactions collectively, you can't change it to Single position management. That is standard system behavior. You might have options though:
1. Reverse all the transactions including the posted flows for this product type and then change to single position management. this option might not be suitable
2. If you want to use the same product type, you would have to create a new valuation area and manage the product as single position management in the new valuation area.
3. Create a new product type all together and manage it in single position management.
Regards,
Nikhil