Every price rises, every category gets renamed, every field eventually needs backfilling. The pattern is always the same: select the records you mean with a criteria set, loop them, write the change. The whole technique is two lines — what matters is the care around them.

The pattern

// ============================================
// MASS UPDATE — run from a scheduled function,
// not from a form workflow.
// ============================================

void Updates.repriceCategory(string categoryName, decimal pctIncrease)
{
    factor = 1 + (pctIncrease / 100);
    changed = 0;

    // Filter in the query. Never fetch everything and test inside the loop.
    for each prod in Product[Category == categoryName && Active == true]
    {
        oldPrice = ifnull(prod.Price,0);
        if(oldPrice > 0)
        {
            prod.Previous_Price = oldPrice;              // keep an audit trail
            prod.Price          = (oldPrice * factor).round(2);
            prod.Price_Updated  = zoho.currentdate;
            changed = changed + 1;
        }
    }

    info "Repriced " + changed + " products in " + categoryName;
}

Video walkthrough

Notes

• Put the filter in the criteria, not in an if. Product[Category == x && Active == true] asks the database for the right rows. Fetching everything and testing inside the loop pulls the whole table into memory and will hit the iteration cap on any real dataset.
• Run it once against one record first. Add && ID == 12345 to the criteria, confirm the result, then remove it. There is no undo on a mass update.
• Keep the previous value. Previous_Price costs one field and turns an irreversible mistake into a recoverable one.
• Scheduled function, not form workflow. Anything touching more than a few hundred rows will exceed the script timeout if it runs inside a form submission.
• Count what you changed. The changed counter is how you find out the criteria matched nothing — otherwise a script that quietly did zero work looks identical to one that succeeded.