It depends on what you mean by "simpler."
The "modulo" solution makes for a short expression, but will increase execution time, because "modulo" is a relatively expensive calculation.
The "ternary operator" (x ? y:z) makes for fewer lines of code in the source, but it harder to read (IMO), and is likely to produce the same code as the if/else statement.
As other have said, your use of Strings is inappropriate and inefficient. Assuming that the argument to your function MUST be a string (ie, cleaning up this function, but not looking beyond it), you can improve things by only doing the string to number conversion once:
float Plane::to360(String dir)
{
float dirval = current(dir); // (or maybe "int" ?)
if (dirval < 0) {
absRollAngle = (360 + dirval);
} else {
absRollAngle = dirval;
}
return absRollAngle;
}