alfattal

As R C-R said, you don't have a Product Key. If you need to reinstall the program, or install on a different Windows 10/11 computer, just login to the Microsoft Store using the same credentials you used for the initial purchase, and you'll be able to download it from there.

If you did purchase directly from Serif, I think there are three files you'll need to make copies of, and to switch between releases I think you'll need to rename all of them because there application will expect the folders to have their standard names.
1. C:\Program Files\Affinity\Photo
2. C:\ProgramData\Affinity\Photo
3. %AppData%\Affinity\Photo
So, if I were setting up 1.10.0 and 1.9.2 so I could choose between them, and I had 1.10.0 installed, I would: 
1. In each of those folders, make a copy of the Photo directory and rename the copy Photo1100
2. Uninstall Photo 1.10, and install Photo 1.9.2.
3. Run 1.9.2 so it updates any needed files in ProgramData and AppData, then close 1.9.2. Note: it is possible you'll lose Assets, Brushes, etc. when downgrading, so be prepared to restore them from a backup.
4. As in step 1, make copies of the three Photo directories, and name them Photo1092
5. Then when you want to run a particular release that isn't the current one, you just make sure the app isn't running, and rename some folders or copy them so they are named Photo rather than Photo1092 or Photo1100, start the app.
Note that this is all theoretical from my end as I haven't tried it yet. 

What Version of Windows are you running? By latest version do you mean the 1.10 ? Where did you purchase Affinity Photo? From the Serif Affinity Store or the MicroSoft Store? If from the MS Store I think your stuck, no way to get an earlier version and I think you will not be able to run 2-retail versions. I think ( I have not tried this), you will need to first uninstall the current release.
Uninstall the current release Install the earlier version. Copy or Move the Program Files>Affinity>Photo folder with all it's contents to another place, preferably another hard drive. Install the current release version. If you get a warning, This will update. Stop there, because it update the older version you just moved. I don't think it will since it should not be in the default location. If you manage to get it installed, then all you need to do to run the previous version is run it from the executable file, Photo.exe. To make it easier to launch the earlier version, create a Desktop Shortcut to it. Rename the shortcut to something like Older_Photo. Like I stated I haven't tried this. I tried a backwards method, where I was going to just install the older version, and tell it where (separate hard drive) to install it. However I was met with the Update warning, so stopped.

I have both the 1.6 and 1.7 released versions on my (Windows 8.1) PC but you really need to have planned this in advance.
First I copied my 1.6 program folder to somewhere else then did the upgrade
To run the 1.6 version I just need to run the .exe file from the folder location I copied it to.
In your case, if you have a backup of your 1.6 program folder you could restore it to somewhere else
Alternatively, you could downgrade your current version then do the same as I did above to have both versions on your PC
 

Try this link please 
https://affinity.serif.com/grade-ui-kit/signup/

@alfattal
Welcome to the Serif Affinity forums 
The Microsoft Store version doesn't offer the grade-ui kit automatically as Microsoft would not allow it, so we are looking for an alternative way to provide it for all owners. When I am back in the office I will have someone post some instructions to you tomorrow as you are welcome to this content.