Boeing has issued changes to controversial control systems linked to two fatal crashes of its 737 Max planes in the past five months, the BBC reports.
But it is still not certain when the planes, which were grounded worldwide this month, will be allowed to fly.
Investigators have not yet determined the cause of the accidents.
As part of the upgrade, Boeing will install an extra warning system on all 737 Max aircraft, which was previously an optional safety feature.
Neither of the planes, operated by Lion Air in Indonesia and Ethiopian Airlines, that were involved in the fatal crashes carried the alert systems, which are designed to warn pilots when sensors produce contradictory readings.
Boeing said that airlines would no longer be charged extra for that safety system to be installed.