Cannot browse upstairs PC from Downstairs

I can see both Windows 10 PC's listed in Windows Explorer (under the Network tree on the left hand panel).
I can browse both computers from the upstairs computer.
I cannot browse the (Ethernet wired) upstairs computer from my downstairs (wireless) computer.
The upstairs computer first asked for my downstairs Windows login, which I gave, and I was able to browse my downstairs PC from upstairs.

There are two accounts on the upstairs computer and I can access both after entering their individual credentials. When I try to browse the upstairs computer from downstairs it prompts for the upstairs computer's username and password (for some reason the name of my downstairs PC is listed as the domain in the popup window). The username/password combo is rejected. I've tried every credential used on the upstairs PC but none are accepted.

I opened the Credential Manager and there are no Windows credentials listed on either PC. I added a credential to the upstairs PC but I still wasn't able to browse it from downstairs.

If I can't browse the upstairs PC from downstairs then I don't want the upstairs PC to be able to browse my downstairs PC. However, I can't find a way to retract whatever permission I gave the upstairs PC. Upstairs users have free reign to browse my User and Public folders.

I'd appreciate any help to allow me to access the upstairs PC or to force the upstairs PC to enter my credentials for access as before. Thanks.

I was able to access the upstairs PC by entering its username syntax as follows:
username = name_of_pc\account_username
then entered password of that username account)