Hi MAbbas,
While in manual learning mode, you should be able to select all learning suggestions and click "Delete Suggestions". This will delete the learning suggestions and enalbe the learning proces to start over again. Don't choose "Ignore Suggestions", because the suggestions will not learn again.
When in Automatic learning mode, you should indeed create a new policy to start over...
This link is really stong documentation in my opinion:
https://techdocs.f5.com/kb/en-us/products/big-ip_asm/manuals/product/asm-implementations-12-1-0/23.html
Hope this helps!