Well, there's 2 ways to update a solution. I prefer to uninstall/reinstall. In which case you can use a new .wsp that is named anything you want.
There's also an update solution command, but, I truthfully have never used it.
My steps are below. I have these saved to a text file on my server's desktop for easy access when it's time to do deployments. In the case of an update, you would start with the 'UNINSTALL' block below, followed by the 'INSTALL' block. Change the .wsp filenames accordingly, of course.
FWIW, I have my wsp files in C:\ and run my commands from there.
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INSTALL:
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stsadm -o addsolution -filename mySolution_v1.wsp
Operation completed successfully.
stsadm -o deploysolution -name mySolution_v1.wsp -allowgacdeployment -immediate
Timer job successfully created. <<-- worked first time around because feature-set was not web-app specific
Later on, I had an updated version of the mySoluion.wsp which made edits to the web.config file
for the entire web application via a feature that was scoped to "Web Application".
When I got ready to install the solution this time around, the same 'deploy' code from above
fails because I did not supply a URL:
stsadm -o deploysolution -name mySolution_v2.wsp -allowgacdeployment -immediate
This solution contains resources scoped for a Web application and must be deployed to one or more Web applications.
Thus, I had to add the URL to my command, like so:
stsadm -o deploysolution -name mySolution_v2.wsp -allowgacdeployment -immediate -url http://intranet/docs/
Timer job successfully created.
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UNINSTALL:
***********************
***********************
FIRST - LOGIN TO SHAREPOINT AND:
>> Go to Site Settings
>> Go to Site Collection Features
>> Deactivate related features
>> Go to Site Features
>> Deactivate related features
stsadm -o retractsolution -name mySolution_v1.wsp -immediate
Timer job successfully created.
stsadm -o deletesolution -name mySolution_v1.wsp
Operation completed successfully.
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UPDATE: (again, I never used this one)
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http://technet.microsoft.com/en-us/library/ff607724.aspx
Update-SPSolution -Identity b0915381-25a9-4b01-bd05-a014500a2764 -LiteralPath <\\path\to\new\solution.wsp>