You cannot delete activities on the watch, but you really don't need to. What you are seeing on the watch history is just summary data, there is no detail behind it and it uses virtually no memory. It is simply the totals for the last 10 activities. When you hit 11 the oldest one gets deleted automatically. Your only option to totally clear it is to do a factory reset.
Unfortunately, you can't edit the GPS data. You can edit the duration and distance which will correct the pace, but the map will remain unchanged as the underlying GPS data is still there.
To edit an activity, you need to go to the activity through the dashboard on MySports, hit the More button on the top right and select Edit. You can then edit distance, time calories, etc. The totals will be correct, but the map will still reflect the entire route as you are editing the totals not the detailed GPS data.
Unfortunately, there is no way to upload activities not created by the watch and no way to manually add an activity. However, you can start a new activity, immediately stop and save it and then upload it. You can then go into MySports and edit the activity with the correct distance, time, etc... The date and time of the activity will not be correct but it will get added into your lifetime totals and other stats.
You can go to MySports and go to your Account (click your name in the top right), go to Settings>Manage My Account and there is an Option there for Manage My Data. Click this and you will be able to download all activities (but you cannot specify a date range).