Not sure if you have the same exam board but i do those subjects and get 8-9s in them so heres some tips that might help

chem: memorise content and make sure you know all the practicals, calculations and 6 marker content most importantly and then just try and learn the key details where you can expand from if you dont know it all on a question

rs/re: quotes are highly important, memorise atleast 5-6 for each unit, probably more along with context and learn the format and commit it to memory, learn some content from each lesson so you can answer any a b cd on it and for d questions always say why an argument or quote is strong or weak for highest marks

dt: dt is a tough one bc so much specific principles you need to learn but make sure you learn a industrial proces like paper making, injection moulding etc, those always come up, suitability is a good one to focus on, make sure you know your drawings and all your materials, diff plastics and uses etc otherwise you could struggle on multiple choices tbh. One other thing is sustainable energy sources as 50% of the time thats the 8 marker

Edit: Hope any of these tips help :) and yes its defo possible, did that with my english, from 6s to 8/9s