how to modify GOPATH in powershell

I am trying to add my project directory to GOPATH, in linux I can do

export GOPATH=$HOME/mygo in ~/.bashrc

what is the equivalence in powershell

This should follow the way you set any Environment variable with Powershell (as described in this article):

If you want it permanent (ie will apply for any future shell):

[Environment]::SetEnvironmentVariable("GOPATH", "C:\Your\Path", "User")

> One thing to watch out for: when we used SetEnvironmentVariable to create a new user- or machine-level environment variable that variable didn’t always show up when we ran this command in Windows PowerShell:

Get-ChildItem Env:

> Or at least it didn’t show up until we restarted PowerShell. (Or started up a new instance of PowerShell.)
However, we could retrieve the value of the new variable at any time by using this command:

[Environment]::GetEnvironmentVariable("GOPATH","User")

If you want it for just the current shell, as mentioned in cmotley's answer and detailed in the same article:

$env:GOPATH = "C:\Your\Path"

As Jaykul comments, setting that line in your %UserProfile%\My Documents\WindowsPowerShell\profile.ps1 is the equivalent of an export in ~\.bashrc:
See "Windows PowerShell Profiles".
(There are actually 4 profiles, the one profile mentioned applies only to the current user, but affects all shells)

Alternatively, you could do this:

$env:GOPATH = "$HOME\mygo"