How can I change default tenant in Microsoft Graph Explorer

If you want to sign into your own tenant in Graph explorer, use tenant query string parameter.

https://developer.microsoft.com/en-us/graph/graph-explorer?tenant=contoso.onmicrosoft.com

A simple url to go to Graph explorer is https://aka.ms/ge, when you land on the destination, just add tenant query string parameter (domain name or tenantId, both work).

If you are already signed into a tenant, you will have to logout from there. currently automatic switch of tenant is not supported.

Create a service account in the tenant you want to access.

1. Create a Service account with appropriate accesses/groups in the Target Tenent.
2. In a clean/Private Window bring up https://aka.ms/ge the Graph Explorer. Note that the test tenant will be engaged.
3. Select the user button and initiate a login with the credentials setup in step 1.
4. Initiate the first Get the Get My Profile to verify the service account logged in is found.

This is fundamentally how Microsoft Graph works, it retrieves data from the tenant you (or your app) authenticated against. It cannot query across multiple tenants.