How to unscrew trim piston seal?

[Yamaha3535]

2003 Yamaha 130.I noticed the seal or dust seal to my trim piston is failing. I’m going to replace the seal but what tool can I use to unscrew the cap screw that has the four holes on top?can anyone recommend the tool or process of unscrewing/ replacing seals?. I don’t want to mess that bad boy up. Also how can I empty the system out to see if water got in thanks

[fairdeal]

the "correct" tool is referred to as a "pin spanner" or "face spanner".
There are many variations, from the simplest with only 2 pins to others with 3 or all 4

several helpful youtubes

one from a tool supplier:

https://www.youtube.com/watch?v=nfPyXNQQOf4&t=7s

one which specificaly addresses water in the unit:
https://www.youtube.com/watch?v=n0WsNGyGlV0

[marchmadness]

I just did four seals with marine tech tool. Good quality tool made it a lot better.

[rodbolt17]

on the showa unit, if the wiper leaks you have other issues. the showa is not a pressure down system.