Unfortunately this is not possible. As soon as you click 'upload', your data hurtles down through our processing engine to create the orthomosaic, DSM, and DTM.
If you've accidentally left some photos out, then recommence the upload. You can always delete the first one.
Otherwise if you're just hoping to add more data from a separate mission on a different day or time, just create a new dataset instead. This keeps everything clean and tidy with the correct day/time/location stamps on the data.