Your "PIN" is an alternative method of signing into your computer (and installing apps) without having to use a "password." (Its way easier to enter 4 numbers on the fly). In order to switch from "Password" use to "Pin" use, you _MUST_ be signed into your Microsoft account first. Then enter the Windows "Settings" and then click on "Account Settings" and from that window you can select options for signing in (other than, but also including "password"). Select "Pin" (aka "Windows Hello Pin") and follow the instructions. After you set it up, you will then be asked to enter your "PIN" _instead_ of a password when you sign into the computer. You will also then use the "PIN" number for your Microsoft Account, and anything else requiring a "PIN".
I hope this was an easy enough explanation. I can tell you from experience that I have never regretted switching to "PIN" for my computer and Microsoft logins. I've been using "PIN" since they began offering it.
Ps... Your inability to "reset" the PIN is because you were not *signed into* Microsoft when you attempted to do it. You _MUST_ be signed into your Microsoft Account before you attempt to change your PIN number.
Best of luck...
Sincerely,
Gregg