1. Back up the data in your phone
For Apple user, you can back up your data to iTunes or iCloud. Do take note iCloud only limited to 5GB of free storage, beyond that, you have to pay for more storage space.
You can follow this guide for step-by-step instruction to back up your iPhone and iPad.
Android user can either use Google account or manually drag and drop the files to your computer. We do not recommend the latter as you may miss out copying important files.