You could start by syringing the reservoir empty, refill with clean fluid, remove return line at reservoir, (usually on the side, bottom one is feed line to rack) fit return line into the top of a coke bottle or similar fluid receptacle, remove fuel pump fuse (stops engine from starting, needed for next step) get someone to crank engine as you add fluid to reservoir to replace fluid flowing into bottle from return line. Continue until you are getting fresh fluid from return line, (I'd have at least a litre or two to ensure you get it all out) Reinstall fuse and return line, check for leaks whilst running; Done!
Good luck!