The process for updating an Ubuntu MATE instance remains the same - you can still upgrade to 26.04 even if there is no official MATE release. That’s because the data remains in the repositories for software versions, etc. independent of the release of ISOs.
Just run sudo do-release-upgrade as normal on the command line.
If you want an update, I suggest you read the following written by Thomas/@teward
The only changes that have occurred relate to the new Ubuntu MATE development team members working on the stonking release (26.10), recent evidence of that can be seen here which will hopefully mean we have a daily or monthly snapshot 2 available soon of stonking (what will be 26.10).
There has been, no change for 26.04, nor is any expected; it’s already been released (Ubuntu MATE didn’t apply for LTS status thus no point release media is expected either). The lack of install media really only impacts ‘new installs’ (and that can be worked around anyway).