I am the seller of a land contract, the buyer is behind, should I take them to small claims court?

No small claims has nothing to do with it

What exactly does the contract say about them being in default.

Normally there should a clause in that that if and when the other party is in default it changes to a rental. But only you know what type of clauses are in your contract.

Normally you will send them notice of the default, give them the time frame to bring it back into compliance.
Then if they don't, you do the default.

Yes, the contract should spell out what happens in case of default. Generally, the contract reverts to a rental lease and you follow the eviction process for a delinquent tenant.