Boeing’s Starliner spacecraft on approach to the International Space Station during an uncrewed test in 2022. Image: Bob Hines / NASA / Flickr 
The original $4.5 billion contract, awarded to Boeing under NASA's Commercial Crew Program, envisioned six post-certification operational flights of the Starliner capsule.
With the latest modification, NASA has designated the remaining two flights as optional.
The Starliner program has been dogged for years by delays, technical faults and cost overruns.
The next Starliner mission will be uncrewed, NASA said, marking the third such test in a development effort that has cost Boeing more than $2 billion since 2016.
Propulsion system issues on the capsule during its most recent mission last year forced its debut crew to remain on the International Space Station for roughly nine months.
The next mission, known as Starliner-1, will deliver cargo to the International Space Station, NASA added. Boeing and NASA are targeting no earlier than April 2026 for the flight.
(Reporting by Nandan Mandayam in Bengaluru; Editing by Krishna Chandra Eluri and Tasim Zahid)