How do I change prices on many products at once?

Use the Bulk Price Update plugin (Plugins). Choose which products to change (all, a selection, or by category, brand or tag), then raise or lower prices by a percentage or a flat amount, or set a fixed price.

By default the change applies to both the price and the sale price; set Apply to to price only if you don't want sale prices changed. Use Preview before applying, because there is no undo.

For a temporary sale, prefer a sale price or a coupon so the original price is kept.