Legacy B4 99 Changing ATF......how?

[kiwilegacyB4]

Have just been into my local dealer and he said I need a new ATF temp sensor . Car is a 99 Legacy B4.........having gear change problems with the tiptronic box and the power light keeps flashing. Saying this, it was a really hot day here yesterday and the problem only came up on one of my journeys.

I have read a bit about similar problems and I want to try a gearbox fluid change first.

Is it easy? Can I do it? What oil do I get and how much will I need?

Any help would be greatly appreciated