First, we would like to express our condolences for your loss.
Second, you will be filing as Married Filing Joint in 2018 - I assume that this is how you filed in 2017. This means that except for the tax law changes that Congress just enacted to start in 2018, your tax return should be similar for 2018 to 2017. So little need to update your W-4.
You will not switch to Single until tax year 2019. The exception is if you are living with a dependent, then you can file Qualifying Widow(er) for two years - 2020 and 2021 (if, of course, tax law does not change in the meantime). QW provides better tax rates than filing Single.
There are other qualifications for Qualifying Widower - please see IRS Publication 17 for more information.
Towards the end of 2018, you might talk to your HR people to see if – based on
your situation and level of withholding – which filing status would make the
most sense for you (Single, Married, or Married but withhold at higher single
rate) on your W-4.
BMcCalpin gave you an excellent explanation.
For 2021 and going forward I recommend filing your W-4 as Single.
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