How to fix a stuck valve on 350 stuck open?

Unscrew the top of the valve and take out the valve. Make sure it is clean and apply valve oil to to the valve. Depending on how stuck it is and if the the euphonium has not been played in a long...

If you mean on the exhaust manifold, penetrating oil and gently tapping around rod and on ends . Over & over with oil !!

you get something very stiff and keep trying to lever it open, if this doesnt work smash the uncscrew the screws and crawl in, if this doesnt work then just brake your trunk

Remove it and try to blow through it. It is more likely carbonized over and stopped up. To check this you have to cause it to open by electrical or mechanical means. Usually only the vacuum...

When mine did that, I tried pushing on it a bit and pressing on it while pushing the button.