Revision mode is a tool you use once your script is in production (or darn close). The purpose of it is to lock the script during production; so Scene 1 remains Scene 1 and Scene 100 remains Scene 100, regardless of whether you added or deleted a few scenes in between.
Once you're ready to use revision mode, just click the lock icon at the top of your script. This will lock you into revision. You can't delete sceneheadings and any scenes you had will be sub-labelled alphabetically: 1A 1B etc.
Your edits will appear in industry-standard colour on the screen. When you print that edited script, each changed lined will have an asterisk on either side so you can easily track changes.
If you're ready to do a second revision, then you can add a second set of marks using the "add revision" plus sign
And if you'd rather delete a scene, you can use the omit button
If you're ready to take the script out of revision, just select the lock icon again and say "yes" to "Reset all scene numbers and revision marks?" This will change those 1As to 2s and get rid of the marked edits, accepting the script as currently written.