In order to change the email address on your Steam Account, you will need to follow these steps:

• Open the Steam client and select “Steam” in the upper left.
• Go to “Settings”.
• From the "Account" tab, select "Change Email Address".
• Choose the option to send a verification email to the email address associated with the account.
• Open your mailbox, and an email from Steam Support will be sent to the email address associated with your Steam account. If you don't receive an email, make sure to check your spam folder.
• Enter the 5 character code from the email and then change your email address. Steam Support will immediately email you a link to click on to verify your new email address. If you don't receive the email, please try using an email address with a different email provider.

Here you can check how to change the email address on your Steam Account: