Can't tell you exactly without TV model number can we?
If the TV is old it may not have a digital ova tuner built in at all. That may be why you can't find a scan feature.
If that is the case you will need an external ATSC tuner with output connection compatible with the TV.
Channel scan is usually in the TV set up menu. There is select whether the connection is over the air or cable. Then run the scan.