A probable cause is your phone has insufficient memory. The Windows 10 Mobile update requires at least 1.4Gb of free
onboard storage available. In order to free up space, you need the Upgrade Advisor app. Download it from the store, and it will assist you with the Windows 10 Mobile upgrade, as well as helping you free up some space by deleting temporary files and suggesting what needs to be done so as to proceed with your upgrade process.
Also, you can manually move stuff from your phone storage into a Micro SD Card, if you have one in your phone. Download and use the Microsoft Files app to do that using your phone.. Alternatively, you can copy it to your computer or upload it to OneDrive and then restore it later onto your phone.
However, if storage space is not the issue, I advise you do a soft reset before retrying to upgrade:
Whilst your phone is on:
Press and hold the Volume Down and Power buttons at the same time until you feel a vibration (about 10–15 seconds). Ignore the "slide down to power off" screen
When the phone vibrates to signify powering up, release the buttons immediately and let the phone restart.
Soft resetting a phone usually resolve most phone and application problems.
After doing so, download the Upgrade Advisor application from the store, and continue to run it. It will assist you with the software upgrade.