modify data at a specific line and columns

program main
         integer ln
         real    newval,oldval
         ln=178
         open(unit=10,file='file.txt',
     &        status='old',form='FORMATTED',access='sequential')
         rewind 10
         do n=1,ln-1,1
            if(n.eq.ln)then
               read(10,20)oldval
            endif
         enddo
         write(*,*)'Old value was'
         write(*,20)oldval
         do n=1,ln-1,1
            if(n.eq.ln)then
               write(10,20),newval
            endif
         enddo
         write(*,*)'New value is'
         write(*,20)newval
   20    format(10x,f10.3)
         close(10)
      end program

in the above program I would like to modify the columns 11-20 in line
178 of "file.txt". Text file contains lines of different lengths,
character strings, floating point numbers etc.

It seems that I am unable to locate ****perly the columns and/or the
line.
Any help will be much appreciated.

regards
MI