Ok, my tank is now processing ammonia and nitrites within 10 hours..I'm getting my fish tomorrow, so when should I do the water change, and is 75-90% really the best amount?
I have a 90litre Fluval Roma

You can do 100% if you like .... the good stuff is in the filter NOT the water ! But I would do 50% first go, so as to keep some mature water in there.
Do this a few hours before adding the fish.

Is it okay to do it now? Im getting my fish tomorrow afternoon, as I'm busy all morning?

Also, is it better to use warm water or cold water with water changes or doesnt it really matter?

Should be ok to do it now.... if you add water at approx right temp, it will save your heater ! dont forget that dechlorinator ! (add this to bucket, or whatever you are carrying the water in) dont dump it in tank... you have a delicate balance of bacteria in there - would be a shame to kiil them now !

Thanks for the help

I would suggest that somewhere around 90% is best. Your tank water will have a massive amount of nitrate accumulated from the fishless cycle, and its best to get rid of it before adding fish.

I do agree that it will be fine to do the water change now. Match the water temp etc as far as possible, but its not absolutely critical. The main thing is to make sure you dechlorinate the fresh water.

Regards

BTT

i know this can be frustrating when you've worked hard to get throught he cycle, but don't rush out to get fish the minute the tank is cycled. i always advise to wait for a week after the tank first processing the ammonia and nitrite in 12 hrs. keep adding your 5ppm per day, if it's done it consistently for a week then do you're big water change and get some fish.

often when a cycle has just finished it's still a bit unstable, you'll occasionally get someone who'se gone through the whole cycling process, got some fish then the cycle crashes a day or two afterwards and they're stuck with fish in cycling after all their hard work to do it right.

bear in mind you may be keeping fish for the next 10+ years, waiting 1 week more to make sure it's safe is no big deal but can make a big difference. so just try and have a touch more patience and don't rush things.

Yeah i think you should wait atleast three to five days before adding fish