Deepali,

you could change the work item type of all work items which have it to something else and then delete. But my guess is that you want those work item to keep the type you are trying to hide.

In that case simple ensure by means of permission that no role (except possibly the admin role) can "Create Work Item" of that type:
- login with project admin rights to https://<YourPublicURI>:9443/ccm/admin
- navigate to manage your project
- go to permissions, choose radio button for "operations"
- in Work item --> Save Work Item --> Create Work Item --> Create Work Item of a Specific Type
choose the type you want to restrict
- revoke permissions for all roles, except maybe a special admin role (you can create for that purpose)
- make sure to save project area

Now regular users will not be able to create a work item of that type.

If this answer is helpful please mark it as accepted.

- Arne