How to update Model attribute in Rails

SO I have this app, that let's users order shirt online.

This is how the item order looks, it has shirt size, shirt color, phone number, address, and order status.

I was modifying the existing code so that the admin users can change the order status from "ordered" into "processing", or "delivered" by selecting an option from a dropdown list beside the order status or to make it easier using a textfield by manually writing the supposed status, and click on submit button.

I get the view part, but I'm having difficulty with the update method in the controller, I don't know where to start being a novice rails coder myself.

SO this is my Model

class Micropost < ApplicationRecord
   belongs_to :user
   default_scope -> { order(created_at: :desc) }
   mount_uploader :picture, PictureUploader
   validates :user_id, presence: true
   validates :shirtSize, presence: true
   validates :shirtColor, presence: true
   validates :contactAddress, presence: true
   validates :contactNumber, presence: true
   validate  :picture_size
   #uncomment picture presence validation in the future
   # validates :picture, presence: true

   SIZE_LIST = [ " ", "S", "M", "L", "XL" ]
   COLOR_LIST = [ " ", "Black", "White", "Gray", "Red", "Green", "Blue", "Navy Blue", "Yellow", "Pink"]

   private

   # Validates the size of an uploaded picture.
   def picture_size
       if picture.size > 5.megabytes
          errors.add(:picture, "should be less than 5MB")
       end
   end
 end

VIEW

<li id="micropost-<%= micropost.id %>">
<%= link_to gravatar_for(micropost.user, size: 50), micropost.user %>
<span class="user"><%= link_to micropost.user.name, micropost.user %> 
</span>
<span class="content">
<%= "#{micropost.shirtSize}, #{micropost.shirtColor}" %>
<% if current_user?(micropost.user) || current_user.admin? %><br/>
  <%= "#{micropost.contactNumber} | #{micropost.contactAddress}" %> 
  <br/>
  <h4>Status: <%= micropost.orderStatus %></h4>
<% end %>
</span>
<span class="timestamp">
Posted <%= time_ago_in_words(micropost.created_at) %> ago.
<% if current_user?(micropost.user) && micropost.orderStatus == "Ordered" %>
<%= link_to "Cancel", micropost, method: :delete,
                                 data: { confirm: "You sure?" } %>
<% end %>
</span>
<span class="content">
<%= image_tag micropost.picture.url if micropost.picture? %>
</span>
</li>

CONTROLLER

class MicropostsController < ApplicationController
before_action :logged_in_user, only: [:create, :destroy, :update]
before_action :correct_user,   only: :destroy
before_action :admin_user,     only: :update

def create
  @micropost = current_user.microposts.build(micropost_params)
  if @micropost.save
    flash[:success] = "Design Posted!"
    redirect_to root_url
  else
    @feed_items = []
    render 'static_pages/home'
  end
end

def destroy
  @micropost.destroy
  flash[:success] = "Design deleted"
  redirect_to request.referrer || root_url
end

def update
  #Have no idea what to do here...
end

private

  def micropost_params
    defaults = { orderStatus: 'Ordered' }
    params.require(:micropost).permit(:shirtSize, :shirtColor, :contactNumber, :contactAddress, :picture).merge(defaults)
  end

  def correct_user
    @micropost = current_user.microposts.find_by(id: params[:id])
    redirect_to root_url if @micropost.nil?
  end

  # Confirms an admin user.
  def admin_user
    redirect_to(root_url) unless current_user.admin?
  end

end