Redeem when the goal arrives, when the fund has stopped being the fund you bought, or when your own circumstances change. Those are the three reasons. A falling market is not on the list.
If the goal is a year away, start moving equity into debt gradually now, so a crash in the final months cannot take a decade's growth. If a fund has changed manager and style and trailed its benchmark for three years, replacing it is reasonable. If you lost your income, redeeming to live on is what the money was for.
Before redeeming for any other reason, write down why. If the reason is a headline or a feeling, wait thirty days and read it again. Most of the time the reason will have passed, and your units will still be there.
Sell for a goal, a changed fund, or a changed life. Never for a headline.