I need to replace an electric shower who do I need plumber or electrician.
Can I do it myself?
You need both trades, you should have a 10mm feed in there. I wouldn't risk it. By doing it yourself.
It is easy enough to do yourself as long as you are confident. If it is a like for like ( the same kW) then you good to go. If you change for a higher wattage, then you need to make sure that you wiring is the right thickness and that you fuse on you main box is the right amp. In this case I would get a qualified electrician
You need both and if you requiring a up grade on your shower you will definitely need to upgrade your electric cable and can only be done a qualified registered part p electrician