I need to know how to replace a broken valve.

You'll need to remove valve cover and rocker arm from offending valve. You will also need to remove spark plug and put in an air plug asyou will need air in cylindr to hold valve in position. Then tap retainer with a hammer or socket and take keepers out and remove spring/springs. Hav enw spring with compressor ready to install, compressnew spring far enought to put retainer and keeper in then unload spring. When finished tap top of vavle with hammer to open and close valve to make sure it stays together then reassemble. If you do not have type of spring compressor they are available at auto parts stores for rent/purchase. You need at least 100 psi to keep spring in head. If it falls through you will need to pull cylinder head off.

Note when tightening bolts it is refering to rocker arm bolts.