Southwest Airlines announced a three-year plan on Thursday to revitalize its operation and customer offerings as it defends against an activist investor calling for new leadership and a strategy overhaul.

The plan expands on a series of changes by Southwest in recent months, including plans to add premium seats, introduce red-eye flights and replace its pick-your-own seating system with assigned seats, starting in 2026.

The airline said on Thursday that it would begin selling vacation packages and was partnering with international airlines, starting with a connection in Baltimore via Icelandair next year. It also announced a $2.5 billion share repurchase program and plans for operational changes, including turning planes around faster and finding ways to save on costs.

“We’re now ushering in a new era at Southwest, moving swiftly and deliberately to transform the company,” Bob Jordan, the airline’s chief executive, said in a statement.

Southwest shares jumped more than 9 percent after the announcement, which came ahead of a day of events the airline scheduled for investors and analysts.

The airline is making the moves under rising pressure from an activist hedge fund, Elliott Management, which has amassed a stake of more than 10 percent, worth almost $2 billion. Elliott has said the airline is underperforming and has placed blame on Mr. Jordan, who has worked at the airline for decades.