how to replace the solenoid valve on triton caselona 8.5kw electric shower?

how do i replace the solenoid valve on my triton caselona 8.5kw electric shower if this is the fault. After the mains water came back on (frozen/burst?) I turned on the shower at cold setting. There was a little air and dirty water and after a short time, water came ,heated up and I used the shower but the water did not stop coming out of the shower head after the power was off. I turned off the water supply, waited a few days tried again but no flow of water only drips. Is the repair quite simple to do or better left to qualified person, is it a plumber or electrician?