Step 1: Download the open-source video transcoder program "HandBrake" (https://handbrake.fr/). This is a free and Multi-Platform program so it will work with Windows, Mac, and Linux.
Step 2: Load up your iPhone movie into the program by connecting your iPhone or iPad to your PC with a USB cable. When asked on your IOS device to Trust This Computer, tap Trust or Allow to continue, next, select all the videos you wish to import.
Step 3: If you do have multiple videos, create a file of all your videos to easily import them into the software HandBrake. Then, in the software, make sure to add each video to the queue. Do this by selecting each video in "Title" and clicking "Add to Queue". 
Step 4: Check your preset settings before converting your video file. To determine the correct preset select Preset (found right under Title) > "Web" > Vimeo YouTube HQ 4K. The numbers that follow this preset are dependent on the frame height and frames per second of the file (ie. 2160p60 4K). You can find the height as the second number of 3840x2160. The important thing is that the preset you select matches the settings. Once you have selected the preset, click "Start Encode", and expect the converting process to take some time.
Step 5:The file will be downloaded as an M4V file, which is technically an MPEG4 file, and will be able to open in Premiere Pro or any other video program that you will use.
If you have any more questions, please reach out to freedmancenter@case.edu