Here's how you do it for Mambo 4.5 v1.09 stable.

This is assuming you want to use a ready-made template, and not create your own....I added a link to making your own, here

1) Download a template ZIP file. You can find over 300 of them to choose from at either http://www.Mamboportal.com or http://www.Mambohut.com. DO NOT UNZIP THE FILE.

2) Log into "yoursite.com/administrator".

3) Click on Site-->Template Manager

4) In the toolbar at the top of the page (under the menu), click on the "New" button.

5) Click "Browse", and locate the template ZIP file on your system.

6) Click "Upload File".

Your template is installed. If you get errors you files haven't got the correct chmod permissions applied. You can upload as many templates as you want, but I recommend picking your favorites and only uploading them. That way, you take up less space on your server, and you don't clutter up your template manager.

Here's how you delete a template:

1) Log into "yoursite.com/administrator".

2) Click on Site-->Template Manager.

3) Click the radio button next to the template you want to delete.

4) Click the "Delete" button.

5) Click "OK" on the alert window.