C,  62  58 bytes

Thanks to @Dennis for saving four bytes!

f(char*s){*++s>32||(s[-1]^=s[-2]^=s[-1]^=s[-2]);*s&&f(s);}

Try it online!

Recursive version (58 bytes):

f(char*s){*++s-32&&*s||(s[-1]^=s[-2]^=s[-1]^=s[-2]);*s&&f(s);}

Try it online!

C,  62   58  54 bytes

Thanks to @Dennis for saving  four  eight bytes!

f(char*s){s[1]>32||(*s^=s[-1]^=*s^=s[-1]);*++s&&f(s);}

Try it online!

C, 62 bytes

f(char*s){for(;*s;)*++s-32&&*s||(s[-1]^=s[-2]^=s[-1]^=s[-2]);}

Try it online!

Recursive version (62 bytes):

f(char*s){*++s-32&&*s||(s[-1]^=s[-2]^=s[-1]^=s[-2]);*s&&f(s);}

Try it online!

C,  62  58 bytes

Thanks to @Dennis for saving four bytes!

f(char*s){*++s>32||(s[-1]^=s[-2]^=s[-1]^=s[-2]);*s&&f(s);}

Try it online!

Recursive version (58 bytes):

f(char*s){*++s-32&&*s||(s[-1]^=s[-2]^=s[-1]^=s[-2]);*s&&f(s);}

Try it online!

C, 64 bytes

t;f(char*s){for(;*s;)*++s-32&&*s||(s[-1]^=s[-2]^=s[-1]^=s[-2]);}

Try it online!

Recursive version (64 bytes):

t;f(char*s){*++s-32&&*s||(s[-1]^=s[-2]^=s[-1]^=s[-2]);*s&&f(s);}

Try it online!

C, 62 bytes

f(char*s){for(;*s;)*++s-32&&*s||(s[-1]^=s[-2]^=s[-1]^=s[-2]);}

Try it online!

Recursive version (62 bytes):

f(char*s){*++s-32&&*s||(s[-1]^=s[-2]^=s[-1]^=s[-2]);*s&&f(s);}

Try it online!