The answer to this question depends on which website software you have.
If you have a WordPress website you should be able to achieve most of your editting requirements using the built in Content Management System (CMS).
You can consult the WordPress help articles here to understand how to edit pages / posts etc...
If you do not have a WordPress website, or a website with a CMS, or if you wish to edit the core theme files (e.g. the PHP / HTML / CSS files) you can modify the site files that reside upon the webhosting server. You can achieve this by following these steps:
1) First you will need to connect to our server from your computer to access your website files.
Typically, most people do this by using an FTP client as it's simple and easy.
You will need an FTP client programme to access the server and then "pull down" the site files. We recommend File Zilla
The next step is to connect to the server so that you can download / upload your site files.
The FTP details on how to connect to your site were sent to you in the "Your Hosting Account" welcome email.
Further explaination is given here, in this article.
2) Now that you have downloaded your website files, you can edit them in a local programme on your computer.
To do this, you will need file editting software capable of working with html / php / css files.
The industry standard is Adobe Dreamweaver, but this can be an expensive choice.
Here is a good article that summarises various choices
3) Edit / Design your site
Once you're happy with the way the site looks, make sure to upload it to the server, and test it thoroughly for mistakes, bad links AND it is advisable to check your site in more than one browser. because what looks good on one browser may not show correctly on another (Internet Explorer is infamous for this!)
The best way to achieve this is to either download all the different browsers (Internet Explorer, Firefox, Opera, Chrome, Safari etc) or check on different computers that have these installed.
If you need any advice, just get in touch.