Please read this guide: How to Increase Maximum Upload File Size in WordPress
The easiest way to figure out the problem is to request error logs from your hosting provider for your website, then examine them around the time when you tried to upload the file.
You will see what limit you have encountered and increasing it will solve the issue.
The settings in PHP that you likely need to adjust are:
upload_max_filesize
– set this to a value > than your backup
post_max_size
– set this to a value > than your backup
memory_limit
– set this to a value > than your backup
max_execution_time
– set this to 0 (infinite)