Yes, you can change the http port to a different port number, that is one way of doing it. Here is a MS article on it.
It is also recommended that you change your OWA (Outlook Web Access) to be using a secure connection (https) instead of a none secure connection (http). By using a secure connection peoples domain usernames and passwords will not be transmitted in clear text between there laptops / home machines and the server. This has the added benefit of moving OWA from port 80 to port 443.
It’s a fairly easy procedure to setup. Edit the website on the exchange server, and select the “Directory Security ” tab. Select the Server Certificate tab. Run through the wizard to create a new certificate. It will create you a text file that you can then use along with a certificate authority (there are many to choose from) to create the certificate. After the certificate is emailed to you run through the same wizard and follow the prompts to import the certificate. Then on the “Web Site” tab enter 443 into the SSL box and click OK. Tell the firewall/router to allow port 443 traffic to your Exchange server and you should be all set.
People will now need to type in the https:// in front of the OWA domain name, to get to OWA.