Generally, for this method of gathering user input, you would use a while loop that iterates until the input is valid.
Something like this:
is_valid = false;
while ~is_valid
A = input('Choose the unit of measure (C for Celsius or K for Kelvin )', 's');
if ismember(A,{'C','K'})
is_valid = true;
end
end
Or, equivalently:
while true
A = input('Choose the unit of measure (C for Celsius or K for Kelvin )', 's');
if ismember(A,{'C','K'})
break
end
end
