With new threats facing the US besides the old Soviet Union the US is now upgrading its main nuclear deterent to better meet these threats:

For 50 years, land-based Intercontinental Ballistic Missiles (ICBMs) have been part of the US primary strategic deterrence capability, the nuclear-armed triad that also includes submarine-launched ballistic missiles and long range heavy bombers.

Although the main target for the US deterrent – the Soviet Union – imploded in 1991, other threats – such as nuclear-armed rogue states and non-state actors – have emerged. To address these new threats, the US Air Force undertook a major ICBM modernization program.

To carry out this program, the USAF awarded a 15-year ICBM Prime Integration Contract (F42610-98-C-0001) in 1997 to a team led by Northrop Grumman. Since then, the team, which includes Lockheed Martin, Boeing, and ATK, has been carrying out a major modernization of the ICBM system to ensure its readiness…  [Defense Industry Daily]

You can read more details about the upgrades at the link.