Changing an e-mail address for a Steam account is not as easy as it should be. First of all, it can’t be done from the web interface.
The e-mail can only be changed from the Steam client that you should have installed on your computer.
If you google “change steam e-mail”, you will get to a page created by Steam that is saying that you need to contact them if you want to change the e-mail.
To change the e-mail for an entire account, from the Steam client, click on “Steam”, “Settings”, notice the three tabs from the first page of the interface that will open.
Click on “Change contact email address”, enter your Steam password and the e-mail address that you will change to (twice, in order to confirm it). Wait around 10 seconds and don’t click on cancel. The interface will get stuck here for 10 seconds, until the “finish” button will finally appear.
You need to confirm the change from the new e-mail address by clicking on a confirmation link.
Notice that you won’t be able to access the account using the old e-mail, all the account powers will be transferred to the new e-mail.
The e-mail address is very important for Steam as they will send a confirmation e-mail with a security code almost all the time you try to login from the Steam client. This security measure is in order to avoid hacking.