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I don't understand. If you change those amino acids and the function of the protein deos not change, how can you tell if those sites are involved in function? If you change an amino acid and the function is not affected, the most likely explanation is that this amino acid is not an important amino acid for the function.

Brian Thomas Foley I got your point. But to understand the importance if I want to mutate suppose lysine. Now my question is that should I blindly mutate it to any other amino acid or is there any guidelines for this??

If the Lysine is catalytic, and you want to prove that by disrupting the function, you would mutate to an amino acid that has different properties. If you want to change it but maintain function, Arginine is the most similar in biochemistry to Lysine:

https://www.ncbi.nlm.nih.gov/Class/Structure/aa/aa_explorer.cgi

Likewise, Serine is the closest in biochemistry to Threonine.

Although I am not sure about the purpose of your mutation, the first thing to follow is to ensure that the amino acid after mutation does not change the overall structure of the protein. For example, common studies in the literature mutates Cys into Ser. And then consider the effect of the amino acid property on its function. I think Creative Biostructure may have related services in its catalog and you can consult them.

https://www.creative-biostructure.com/custom-mutagenesis-services-341.htm