To look up what's in the backup file, use:

RESTORE HEADERONLY FROM DISK = 'c:\data\yourbackupfile.bak'

To see the names of the database files, use

RESTORE FILELISTONLY FROM DISK = 'c:\data\yourbackupfile.bak'

To overwrite exisitng database from a backup of different database you need REPLACE option. Something like:

RESTORE db_name
FROM DISK = 'c:\data\yourbackupfile.bak'
WITH
REPLACE,
MOVE 'logical_data_file_name' TO 'physical_path',
MOVE 'logical_log_file' TO 'physical_path'

See BOL for syntax details.