In a recent case called M.V. Transportation, the NLRB ruled that employers can make changes to a union contract, even if the union objects. Previously, the precedent that had been established by the board was that in order to make contract changes, the employer had to have clear permission from the union. In this more recent case, the board established that changes can be allowed if the changes to the contract could reasonably be inferred as allowable according to the “plain terms” of the contract.
See "NLRB makes it easier for employers to change labor contracts without union permission", Sean Higgins, The Washington Examiner, September 11, 2019