Check there is actually a fault with the old solenoid (or solinoid if you must)
Buy a new one
https://www.japgmowers.co.uk/westwood-w6-w8-w11-w16-gazelle-ride-on-mower-starter-solenoid-part-1530-1204-7488-p.asp
Compare to old one.
Check you have spanners and screwdrivers that fit the various fastenings and terminals
Check which wires go where and make a note on paper. Label wires if necessary
disconnect battery
Carefully undo terminals
Consider checking old solenoid if you have still not confirmed it is faulty
undo fixing bolts
lift away old solenoid (or solinoid if you must)
Consider checking old solenoid if you have still not confirmed it is faulty
Fit new solenoid
Re-attach cables
Reconnect battery
Test
vdsvds
October 2017