Open the Sessions list and tap View on the session you want to fix — that takes you to the session's detail page, which is where all editing happens. (History also lists your past sessions, but tapping into it opens an event summary, not the session editor.)
A few things are locked in once a session exists, and there's no edit control for them anywhere in the app:
To delete a single roll, open that roll's editor and tap Delete (trash icon). It removes the roll immediately — there's no confirmation prompt, so double-check before you tap it.
To delete an entire session, go to that session's detail page and tap the trash-icon button. This time you'll be asked to confirm: "Delete this session? All shooters, rolls, and bankroll history for this session will be permanently removed. This cannot be undone." Confirming wipes every shooter, roll, and bankroll entry that belonged to the session, all at once — there's no way to recover it afterward.
Nothing about your stats is cached. Hot/cold numbers, roll frequency, personal records, and your DICE/BANK scores are all recomputed live from whatever rolls and bankroll data currently exist, every time you view a screen. So once you fix a mistapped roll, correct a bankroll amount, or delete a roll or session, every dependent stat updates automatically the next time you look at it — there's no separate "recalculate" step, and nothing goes stale in the meantime.