You can book a seat in private traveller from chandigarh or delhi as per the iteniary you reaching from mumbai... Then at kasol if you do not have any bookings... you can further ask for the options...
You can fly to Chandigarh or Delhi by flight. Then you take a cab or bus to Himachal. However, now please make sure you have negative Covid report to enter Himachal as that is mandatory.
Well u can choose flight ,train ,bus as the basis of availabilty ..but i suggest u to choose flight fron there u will departed and arrived at delhi or may be u can get direct flight and yes make sure u have corona test report for better experince withiut any prblm and u will be great to see and experince the beauty