Error:

Append format to the mutable string

[mutableStr appendFormat:@"'%@'", str];

Append string to the mutable string

[mutableString appendString:@"Another string"];

Delete characters from the mutable string

// delete first 10 characters
[mutableString deleteCharactersInRange:NSMakeRange(0, 10)];

Insert string in the middle of the mutable string

// prepend mutable string with "str"
[mutableString insertString:@"str" atIndex:0];

Replace substring of the mutable string with another string

// replace first 10 characters with "str"
[mutableString replaceCharactersInRange:NSMakeRange(0, 10) withString:@"str"];

Replace all occurrences of the string in the mutable string

// Case-insensitive search over whole string
[mutableStr replaceOccurrencesOfString:@"foo" withString:@"bar" options:NSCaseInsensitiveSearch range:NSMakeRange(0, [mutableStr length])];

Set value to the mutable string

[mutableStr setString:@"New Value"];