When a patient's chart is locked. You can see this at the top of the record in the top bar next to date of service.
You can unlock this from the admin section of the EMR by selected the 'manage records' option. see highlighted in the following image:
In the manage records dialog you can search for your patient in the search box by mrn or by name. Once you have the patient selected you will see a list of their records and can unlock the one in question.
Once unlocked, open or reload the record (if it's open already) and you should now see that the option to lock the record again is there.
Now you can make changes to the record as you normally would.