Update self-update.md

[swaroopRajwal]

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I came across this repository, and tried everything it was mentioned here Nothing worked Then I just typed random commands on my ubuntu shell This worked Hope the maintainer of this repository will accept my pull request

[Haroenv]

this is already the suggestion right above though

[swaroopRajwal]

this is already the suggestion right above though

Oh, sorry I thought there's difference between installing yarn again and updating yarn. Thank you.

[Haroenv]

yes, installing will update too