1993yamaha 600 starting up just spins and no engagement to start. How do you replace the starter clutch

Wow. Probably the positive and negative connections have gotten reversed in the solinoid and starter motor. Still stuck? I recommend a DIYer's repair manual that shows electrical connections for the starting system.

low battery would cause it too if unable to spin starter fast enough, or starter clutch

Lets start with the basics...
1. be sure your battery is in good shape and fully charged ( at least 12.6 volts)
2. starting procedure- ignition key to 'on', tranny in nuetral, right hand kill switch to 'run', pull clutch in fully and hit starter button.
3. does it do anything? clicks?... If you get nothing then the problem is likely in the clutch switch, kill switch, or the starter button. You will need a wiring diagram and volt meter to trace down where the power is lost. If you are getting 12 volts to the starter but its not engaging then you can suspect the starter/ solonoid.

PS... My clutch switch has been going bad for a few years now ( Im too lazy to replace such a trivial thing as a safety switch ;). Sometimes I need to hold the start button and work the clutch till the starter engages.

Bad switch or Solenoid. depending on the bike.