How to remove milestone in gitlab.com

i am using gitlab.com and i am new in gitlab,
i have many projects in gitlab.com and i create may milestone also,

but all milestone is created under oldproject and i oldproject is deleted since i cannot find the remove button for milestone i have created.

currently oldproject is deleted but all testing milestone is still there,
so how to remove all the milestone, i still can not find the way to do this.

Maybe you could use the API for that. There is a method to delete milestones from projects
https://docs.gitlab.com/ee/api/milestones.html#delete-project-milestone