How to update y 25H2 to 26H1

Depending what build you're on now, 26H1 may or may not be easily available to you. But here's the thing. It's easy to find out. Assuming you are already enrolled in the Windows Insider Program, just follow these steps:

Start > Settings > Windows Update > Windows Insider Program

Now check to see that you have Beta (or I believe also Experimental would work) selected, toward the top of that page.

Now click on the pulldown "Advanced options." <----- (Some people seem to miss this. step)

There you will find "Windows 11, 26H1" as one of the optional selections. Select that.

Then reboot and do a windows update again. That should bring you up to the 26H1 build

Or, like Johnny said, just wait for 26H2 to appear later on.