Hi, Aman & Michael.
That's correct - if attribute is present on user's entry, you should be able to change it with SCIM. So, for example, for "oxExternalUID" to be mutable with it, follow next steps:
1. Move to "Configuration" > "Attributes" and click "Register attribute" button
2. For "Name" and "Display name" use "oxExternalUID", type - "text", then set "Include in SCIM extension" checkbox; the rest of the fields can be left at default/blank values, or any value can be put there if its mandatory
3. Click "Register" button
From now on, you should be able to manipulate this attribute with SCIM, as long as request is correct. Here is example of request that updates this attribute:
```
{
"schemas": [
"urn:ietf:params:scim:schemas:core:2.0:User",
"urn:ietf:params:scim:schemas:extension:gluu:2.0:User"
],
"urn:ietf:params:scim:schemas:extension:gluu:2.0:User": {
"oxExternalUID": "lsd:afjljflsajfsafjas21312-cngd"
},
"displayName":"test user one scim-upd"
}
```
Hope this helps.