If you do not have the original blueprint file you can make a new one and modify it...

Blueprint updating relies on reusing the <WorkshopID> element in the bp.sbc file. If you know the URL of the blueprint you wish to update take the ID and replace the <WorkshopID> element with that ID number. Then publish that blueprint like the first time.

PROCEED WITH CAUTION. YOU MAY WANT TO MAKE ANOTHER COPY OF YOUR BLUEPRINT. MODIFYING THE FILE INCORRECTLY COULD POTENTIALLY MESS UP YOUR STUFF. Just read, it's pretty simple.

Step by Step.

1.) Open your workshop items. In steam on the second menu bar (STORE, LIBRARY, COMMUNITY, <YOUR NAME>) hover over <YOUR NAME> and select Content from the context menu. Click on the Workshop Items tab.

2.) Find the blueprint you wish to replace and open it. Take the last set of digits in the steam URL should look like http://gtm.steamproxy.vip/sharedfiles/filedetails/?id=**386205468**. Your numbers will be different it is a unique ID.

1.) Navigate your file system to the blueprints folder/directory %AppData%\SpaceEngineers\Blueprints\local\

2.) You should now see all of the blueprints you have created. Open the folder/directory for the blueprint you want to use to replace the workshop item we located above.

3.) You should now see a file named bp.sbc and thumb.png.

4.) Open bp.sbc with notepad or your favorite text editor.

5.) CTRL+F to find and search for <WorkshopID>

6.) You will want to replace the number between <WorkshopID> and </WorkshopID>. It should be 0 if you have never uploaded the blueprint to the workshop. It will show another random number if you uploaded it trying to replace your current workshop blueprint.

YOUR NUMBER SHOULD BE DIFFERENT THAN THE EXAMPLE
In my case after editing it looked like this <WorkshopID>386205468</WorkshopID>
YOUR NUMBER SHOULD BE DIFFERENT THAN THE EXAMPLE

1.) Open your blueprints (F10).

2.) Find the blueprint you just modified and click the Details button.

3.) From the Details window click the Publish button. You should be able to modify the exploration tag at this time.

When viewing your original blueprint you should see

2 change notes ( view ).

You can click the ( view ) link to make notes about the changes to your blueprint.

---- Update sometime in 2017 ----

An additional file was created in the blueprint folder named bp.sbcPB, it looks like some type of binary file most likely used to compress the data saving some bytes on the upload processes or doing some streamlining with the workshop.

If you modify the human readable bp.sbc xml file you will need to delete the bp.sbcPB file for the workshop ID to be changed.
Thank you u/therealslimshady1234 for bringing the change to my attention!

thanks to Stratus for figuring this out!
all credit goes to https://gtm.steamproxy.vip/id/StratusGaming2


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Originally posted by https://gtm.steamproxy.vip/id/StratusGaming2:

I figured it out!

Ok ok... So everything from before is the same except there is one more line that needs the asset ID.

<WorkshopId>*insert asset ID*</WorkshopId>
<WorkshopIds>
<WorkshopId>
<Id>*Insert asset ID*</Id>
<ServiceName>Steam</ServiceName>
</WorkshopId>
</WorkshopIds>

The workshop ID code will look like this as for 2023. I've marked both locations for the asset number to go. Hopefully this will work for everyone.