New version of gem not reflecting in gemfile.lock of an engine

Question

I'm updating the rails version of my application from '4.2' to '6.0'. There are multiple engines in my application. The new rails version is not reflecting in the gemfile.lock of engine which is causing the following error on running bundle update rails command:

Bundler could not find compatible versions for gem "rails":
  In Gemfile:
    rails (>= 6.0.3, ~> 6.0)

    admin was resolved to 0.0.1, which depends on
       customer was resolved to 0.0.1, which depends on
          emails was resolved to 0.0.1, which depends on
             rails (~> 4.2)

Does anyone know how to resolve this error or simply how to update the rails version in gemfile.lock of an engine?