Updating a Log
If the log you want to edit is a Pending Log in Cachly, go to More > Pending Geocache Logs, find the cache you want to edit, and tap that cache in the list. You can now edit the log message text, add an image within the message text, change the log from Found it to Didn't find it, etc. Once you are done making your changes, tap Save in the top right and your update will be saved and eventually sent to geocaching.com when you submit the log.
If you'd like to delete or update a log entry and it's not a pending log, you need to go to the Logs Tab. Locate your previously posted log in the list and tap the ••• actions button on the right of the cache name. You will be able to select Add Images, Edit or Delete.