Within HubSpot's COS blogging platform you can easily manage your Blog Topics. To delete any of your Blog Topics (or tags, if you like), please navigate to Content > Blog > and click on the Topics link.
Once you're in the Topics page, hover your mouse over the topic you wish to delete. Click the Delete button that will appear towards the right (If you don't see a Delete button appear, click the gear that is in its place and then click Delete). When clicked, this icon will provide you with an option to delete the topic from your HubSpot COS blogging platform.
If you wish to detach a particular topic from a blog post instead, please navigate to Content > Blog > access the specific blog post in Edit Mode. Once you have accessed the blog post in Edit Mode, navigate to the Settings Tab, and under Topics you'll be able to remove topic(s) the blog post is tagged with by simply click on the "x" each topic contains.