Can a seller cancel or revise a real estate contract after it was sign by both parties, and the inspection and appraisal have completed?

If you think the seller will try to back out, you need to contract a lawyer.  It would be a breach of contract but you need to plan how you will handle matters with the assistance of counsel.

Like any other contract, a real estate contract may only be revised or cancelled if--

1) Both parties agree to the modification or cancellation.

2) There is some clause in the contract itself allowing its modification or cancellation under certain circumstances--for example, the "finance contingency" (a/k/a "mortgage contingency") which can let a buyer cancel without penalty if he/she cannot obtain financing.

3) If one party violates a material (or important) provision of the contract, the other party may then be entitled to terminate the contract without penalty--for example, if the seller cannot or will not close on the closing date, the buyer could walk away.