If the desired titleblock is already in the drawing it's a piece of cake. If not,... a slightly smaller piece of cake.
If it is in there, look at each sheet in the model tree and find the titleblock listed.
Right click on the titleblock and select delete. Now go above to Drawing resources, find the new titleblock, right click and select insert.
If it is not already there, open a drawing that has the titleblock you want. Find it in the drawing resources as shown above, right click and copy to clipboard. Now go back to the drawing you want in ti live in, right click on titleblocks in the Drawing Resources and paste. It is now available for insertion.
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