Ali, here is more reliable solution of converting the relatively wide range of square wave into sine.(more reliable than my previous suggestion)

Lets suppose you have a VCO that produces sine wave (within the range of your input square wave frequency). If you apply this to a one input of a Phase Discriminator like 4046, and if you apply the square wave( desired to be converted) to the second input of the 4046, you will have DC error voltage which urges the VCO to run at the frequency you have applied as square wave.
This method is called PLL (Phase Lock Loop)

Regards.

Rustu