How to remove outdoor drain cover to clean insides and prevent overflow?
Either stick a screwdriver/crowbar in the gap round the edge and lever it up, or just stick a crowbar into the grillwork and lever.  Be careful with the second option though - if it's fixed in hard, you might snap the grill.

Check the small holes at the top and bottom of the picture - they might contain some sort of bolt which needs to be unscrewed with (eg) an Allan Key.