Are you wanting to clear the old downloads? If so, go into rom manager > menu > clear download cache. If you are refering to the backups made through rom manager, they are stored on your sd card. If you are referring to the old updates in rom manager, I believe once a new update comes through it replaces the old. To my knowledge you cannot go to an old update or download. The only way you could go to an old download (as in a rom or goodies) then you would have had to make a backup and you would go to that instead.
sd card/clockworkmod/cache (or downloads).....you can either plug your phone into your computer to do what you need or use something like astro file manager. The backups do take a up a lot of space, I went in and just kept about 3 that I knew worked well