DIM i
DIM z$

z$ = "   We'll   replace   all   spaces   with   underscores  "

WHILE INSTR(z$, " ")
  i = INSTR(z$, " ")
  IF i THEN
    z$ = MID$(z$, 1, i - 1) & "_" & MID(z$, i + 1, LEN(z$))
  END IF
  PRINT z$
WEND

Result:

_  We'll   replace   all   spaces   with   underscores
__ We'll   replace   all   spaces   with   underscores
___We'll   replace   all   spaces   with   underscores
___We'll_  replace   all   spaces   with   underscores
___We'll__ replace   all   spaces   with   underscores
___We'll___replace   all   spaces   with   underscores
___We'll___replace_  all   spaces   with   underscores
___We'll___replace__ all   spaces   with   underscores
___We'll___replace___all   spaces   with   underscores
___We'll___replace___all_  spaces   with   underscores
___We'll___replace___all__ spaces   with   underscores
___We'll___replace___all___spaces   with   underscores
___We'll___replace___all___spaces_  with   underscores
___We'll___replace___all___spaces__ with   underscores
___We'll___replace___all___spaces___with   underscores
___We'll___replace___all___spaces___with_  underscores
___We'll___replace___all___spaces___with__ underscores
___We'll___replace___all___spaces___with___underscores
___We'll___replace___all___spaces___with___underscores_
___We'll___replace___all___spaces___with___underscores__