There are two names involved, the name of the computer and the name of the user. I think it is the Username which is annoying you. The computer name is pretty well irrelevant - most people don't know what theirs is.
It is very difficult to rename a user properly, because there are a lot of folder names and registry entries involved. **
It is much easier to create a fresh user account with the correct name, copy the right files to its profile, then delete the account with the duff name.
The fresh account can be created under "Start Menu", "Settings", "Accounts", "Other Users". You can decide whether the user is tied in with a Microsoft Account (it's optional, but they try anyway).
Once the account is created, select it and "Change Account Type" to make it an Administrator. This is almost certainly the account status you are used to.
You can then log off and log on again with this "correct" user name (and password, if you set one up). Because there is now more than one account, you are given a "Logon" screen when you start up.
Once logged in to your "proper" account, use File Explorer to copy the contents of the folders under C:\Users\OEM to C:\Users\<proper name>. You should be asked for permission to get access to the "other" user's files.
Once you are happy that everything is working properly for you under your "proper" name, you can use "Start Menu", "Settings", "Accounts", "Other Users" to remove the "OEM" account.
** In a large organisation, a name change, such as a woman with a new surname after marriage, usually ends up with the folder names being left alone because of the effort involved.
