If you mean "can anyone can change the code you run without your permission?" the answer is: no if you use a non-custodial wallet, yes if you use a custodial wallet.

If you mean "is the lightning network protocol set in stone?" the answer is no. Protocol changes are proposed regularly and wallet makers tend to implement them in a gradual manner. The mechanisms for upgrading the lightning protocol are similar to how bitcoin updates get approved, except in lightning there are fewer core developers and its stability is not as important as bitcoin's. These things make it easier for changes to happen quickly but it also means there is more trust placed in core developers.