。
She has changed quite a lot since then.
Literal
She [topic-は] that time [from-から] considerably changed.
その時から marks the temporal source — 'from that time' — using the same から that elsewhere marks spatial source or cause. ずいぶん ('considerably, quite a lot') is a versatile intensifier — slightly stronger than とても ('very') and with a touch of subjective surprise: 'more than I'd have expected'. 変わる ('to change') in plain past 変わった captures a completed change, with the implicit comparison to a former state. Japanese leaves the 'compared to before' implicit — the listener fills it in from から.