I wish to remove an old forum theme. How can I do so. I dont see anything do so this.

ACP > Customisation > Themes >

On the right hand side of the theme in question click the downwards arrow and choose "Delete"

Note you won't be able to delete the theme set as default, but if that's the case simple choose another default first then you'll be able to. If you're not sure on how to do this, its the middle 'edit' button the 'pencil' type icon you see. Click that for the theme you want to set as default, the on the page that opens you'll see the options there to adjust it as required, at a minimum simply switch on the button I've highlighted in the image below. Remember to save the changes.

As a sidenote: It is actually almost the same procedure for earlier versions if you're using one of those.

Uninstalling a forum skin may not remove the files from your server. From time to time, I find that I have to physically delete the files from the server my site is hosted upon but that I'm very careful about deleting files to ensure that the forum skin files I'm removing are from the skin/template I removed from my community.