Hi Tom
When you drag and drop a clip onto the timeline there are essentially two possible edits that could be 1) an “Insert” edit, and 2) an “Overwrite” edit … you appear to be doing the former, an “Insert” edit, when you want to do the latter, an “Overwrite” edit.
The type of edit that is performed is controlled by where, within the vertical height of the video track that you drop the clip … drag a clip into the timeline but don’t drop it yet … now study the cursor as you hover with the clip over the video track, note how the cursor changes between a right pointing arrow (when hovering near the top of the video track) and a down pointing arrow (when hovering near the bottom of the video track) … the right pointing arrow means that dropping the clip at that position will act as an “Insert” edit, the down pointing arrow means that it will act as an “Overwrite” edit.
Hope it helps
Andy