ive been doing some races online where i want to go flat out for about the first half of the race(R3 or R4 tyres), and then all the rest just making sure i get to the finish line(R2 tyres)

You can change tyre type in the pit instructions box (f12).

Your choice of tyres for 'go really fast' and 'be safe' seems to be a bit reversed, wouldn't you normally choose R2's to 'go really fast' ?

but going fast in r2s in my setups burns out the tyres really quick

and how do you change in the pit instructions?
thanks

In th new patch there is almost no need to use anything other the R1's or R2's depending on the car. If you are finding they are melting then you need to be smoother or improve the setup a bit.

You should stick with the softest compound you can get away with until tyre temperature reaches 100C, which is around the point where harder (cooler) tyres will have more grip than hotter softer tyres.