Answer by Larry Silverman for Is there a way to permanently set the font and window size in PuTTY?

If you have multiple saved sessions and you wish to change them all at once, there is a way.

Open the Windows registry editor by running regedit.exe (requires privilege escalation).
Navigate to the registry key HKEY_CURRENT_USER\Software\SimonTatham\PuTTY
Right click on the PuTTY registry key and choose Export. Place the file wherever you please. Name it something like putty.reg. Very important, make sure you have only exported this one key, and not the entire registry. You might like to place this .reg file in Dropbox or other cloud storage as a backup, or to copy your session info to other machines.
I recommend now making a duplicate of the original file (in Explorer, copy then paste in-place) for diffing or other comparison before you load it back in.
Open putty.reg. Use search/replace to fix entries. If certain sessions do not contain the overrides from default you desire, add them.
Save putty.reg.
For safety, I recommend using a diff tool to compare putty.reg with the copy you made to ensure you didn't change anything you did not intend to.
Finally, double-click putty.reg in Explorer to overwrite the existing registry settings.