You can override Optimizely Forms views as shown here https://docs.developers.optimizely.com/content-cloud/v1.2.0-forms/docs/configuring-optimizely-forms by extracting the zip from the Nuget package EPiServer.Forms in to the ~/Views/Shared/ElementBlocks location. From here there's a FormStepBlock.cshtml which you should be able to rework to your liking

Thanks so much, Scott. 

Hi @Abeer,

FormStepBlock is only for rendering the title and desc of each step.

 <h3 class="FormStep__Title">@Model.Label</h3>
 <aside class="FormStep__Description">@Model.Description</aside>

But If we want to customize the whole stepper, for example like a stepbar or progressing bar, you have to change 'FormContainerBlock.cshtml' localed in ElementBlocks\Components\FormContainerBlock\FormContainerBlock.cshtml.

Checkout the code block around this part:

                if (Model.ShowNavigationBar)
                {
                    <nav role="navigation" class="Form__NavigationBar"  data-f-type="navigationbar" data-f-element-nondata>
                        <!-- btn default class Form__NavigationBar__Action FormExcludeDataRebind btnPrev -->
                        <button type="submit" name="submit" value="@SubmitButtonType.PreviousStep.ToString()" class="btn btn-warning"
                                @prevButtonDisableState data-f-navigation-previous>
                            @Html.Translate("/episerver/forms/viewmode/stepnavigation/previous")
                        </button>

                     @{
                    // calculate the progress style on-server-side
                    var currentDisplayStepIndex = currentStepIndex + 1;
                    var progressWidth = (100 * currentDisplayStepIndex / currentDisplayStepCount) + "%";
                        }
                        <div class="Form__NavigationBar__ProgressBar" style="width:800px; ">
                            <div class="Form__NavigationBar__ProgressBar--Progress" style="width:@progressWidth; background-color: #f47320; border-color: #f47320;" data-f-navigation-progress></div>
                            <div class="Form__NavigationBar__ProgressBar--Text">
                                <span class="Form__NavigationBar__ProgressBar__ProgressLabel">@Html.Translate("/episerver/forms/viewmode/stepnavigation/page")</span>
                                <span class="Form__NavigationBar__ProgressBar__CurrentStep" data-f-navigation-currentstep>@currentDisplayStepIndex</span>/
                                <span class="Form__NavigationBar__ProgressBar__StepsCount" data-f-navigation-stepcount>@currentDisplayStepCount</span>
                            </div>
                        </div>
                        <button type="submit" name="submit" value="@SubmitButtonType.NextStep.ToString()" class=" btn btn-warning"
                                @nextButtonDisableState data-f-navigation-next>
                            @Html.Translate("/episerver/forms/viewmode/stepnavigation/next")
                        </button>
                    </nav>
                }