iOS 10.3 sounds like a minor update to most people, but it is actually big step and contains a big change. Before you upgrade you may want to read this.
Apple has changed the file system from HFS to a new Apple File System that may change your phone in more ways that you expect. Although it is a huge improvements and ultimately gives you more storage and stronger encryption, the two file systems are not compatible with each other. There is no way to convert back to HFS once you have upgraded without blowing away all your data and reformatting your storage and device. Ultimately, once 10.3 is installed, every file stored on your device will be converted to the new format, meaning there are many possibilities for something to go wrong and data to be lost.
Therefore, the solution is simple. Before upgrading your phone, make sure all your information is backed up so even if data is lost, it is easy to retrieve.