UFE 2.5.1 Changelog

What is a 「fixed projectile impact mirror」?
Is it mean a projectile reflection like  Soul Reflect that the SF character-Rose has?

Is it not possible to have one update a month that fixes the most urgent problems? too much time passes between updates, even a year passes. A person could also finish the game and finish it with the problems inside.

You underestimate the complexity of these tasks.
Priorities aside, updating all the licenses and documentation is a time consuming process prone to many errors, and without spending time to properly run tests, we can end up creating more bugs.

Also, as you can imagine, when a report is in fact confirmed to be a bug (and not some overlooked option), I encourage people to debate and try figuring out solutions before a patch arrives. Community input is important and it helps to speed up the fixing process.

As for update cycles, please understand that this is a relatively small business on an niche market. UFE is not a "live service". Aside from symbolic Patreon donations, we must rely solely on sales to keep updating and providing support (regardless of purchased date). Most asset developers (me included) can't rely on this kind of income and often need daytime jobs, so please, be patient.

Use version control tools like GIT (with UI tools like SourceTree) to identify each change. These links should help you:
What is version control?
Git - Getting Started
SourceTree Website