Hi
Editing the text/images is straight-forward, i worked on a couple of trailer templates before. When you open the AE file (composition) you’ll see a timeline with multiple layers, you need to find the layers with the letter T (see 1st attached image) double click and change the text. Same goes for the images. These layers will probably be Pre-Composed just double click and they will open up and you can edit what’s in them. Pre-Composed have an icon see 2nd attached image. Adding music track is just drag and drop on timeline, you may need to adjust length depending on how long the video is. If you need more info let me know
A quick way to replace the images and music would be to find them in the ‘Project’ panel, right-click and select ‘replace footage > file’. If you aren’t sure which one to replace, find it in the timeline, right-click and choose ‘reveal layer source in project’ then do the above. Just be careful with image sizes. i.e. don’t replace a small image with a huge one etc.