Hello and welcome to the forum.
Continue to do water changes until you can get the ammonia and nitrite down to .25 ppm or below. Obviously, you want them both at zero but that will take some time. What are your exact readings for ammonia, nitrite and pH (test your tap water too as the problem could be there)? The pH isn't something you need to worry about getting down (or up). The main thing is that it is stable. Since you say you can't get it down, I'm assuming it's in the upper 7s which does make the ammonia more toxic.
Since you say the ammonia is on the highest level (liquid or strip tests - strips are terribly inaccurate), I would do a 50% WC as soon as possible. Contine to do them every 3 or 4 hours until you get the levels down. After that, you can cut back to 25% as needed to keep the levels down. The only reason not to do WCs that large is if the tap pH and tank pH are significantly different, thus potentially causing a big swing in the tank pH.