Sign cleaning

[CobraPowerHouseWashn]

I go this cleaning job to do and they want there sign cleaned.. Look at these photos and any advice on how to clean this would be helpful.. the yellow stain is my main concern.. Thanks in advance..
The sign feels like vinyl or screen print

 

[Rapid HotClean]

My $0.02. That appears to be a laminated sign and the yellowing is over the entire sign area but shows up the most in the white areas. Could be glue, but IDK.

Everything man made has a shelf life. They probably need a new sign.