code: public CustomAttribute changeCustomAttribute(final Object userIdOrUsername, final CustomAttribute customAttribute) throws GitLabApiException {
  
          if (Objects.isNull(customAttribute)) {
              throw new IllegalArgumentException("CustomAttributes can't be null");
          }
  
          //changing & creating custom attributes is the same call in gitlab api
          // -> https://docs.gitlab.com/ce/api/custom_attributes.html#set-custom-attribute
          return createCustomAttribute(userIdOrUsername, customAttribute.getKey(), customAttribute.getValue());
      }
docs: Change custom attribute for the given user
  
  @param userIdOrUsername the user in the form of an Integer(ID), String(username), or User instance
  @param customAttribute the custome attribute to change
  @return the changed CustomAttribute
  @throws GitLabApiException on failure while changing customAttributes