Wells are lined with a "casing," a large diameter steel or PVC pipe with perforations in the sides. A cap or sanitary seal prevents dirt and debris from entering the well at the top of the casing. A seal has a rubber gasket but a cap does not. A pipe and wires come come through the top of the cap or seal. The seal or cap may need to be removed for repairs or disinfection.
Turn off electrical power to the well at the breaker and determine if your well has a seal or cap. It may have a well cap, which is bolted onto the top of the casing or a cap with a threaded "plug" for access and set screws to attach it. A well seal is on top of the casing and has a rubber gasket inside it.
Call a professional if you have a well seal. Well seals usually have screws or bolts that compress the seal and if extreme care is not taken when removing them, a portion of the seal or the screws could fall into the well. If you believe you can do it without the assistance of a professional, loosen the screws a little at a time, but not too much. It is imperative that you do no remove them completely. Try to pry the seal off very carefully with a screwdriver.
Use a wrench to loosen any bolts if you have a well cap. These may be along the outside edge of the top of the well cap. Alternatively, there may be set screws on the side of the cap, for which you can use a screwdriver. Carefully lift wires or wire nuts and push to the side. Well caps often become rusted onto the casing and require some manipulation with a screwdriver to loosen them. Do not pull or use force. This is a good time to check for any damage or loose connections.