If you are looking to make some major changes to a page within your website then you need to think carefully before starting. Firstly ensure that you have a full, recent back up of your website that you can easily upload back to the server should you do something that crashes your site. If you have another company that looks after your sever, you may need to ask them to do this so make sure you do any changes at times when you can get hold of them should you need to ask them to revert to a back up.
If you are using a content management system, you may wish to duplicate the page first. You can rename the URL and then use this as your test page to make the relevant changes. You can often password protect this page meaning no one can visit it whilst you are in the developmental stages. This will also mean you can share the page with other specific people to view before setting it live. Once you are happy with all the changes, you can copy the code across to your existing page and update it to set the changes live. You can then remove the test page or set it to draft.