You can change the values by putting 1.1 in an empty cell somewhere
(this is equivalent to 110%), selecting that cell and clicking on
<copy>. Then you should select all the values in your sheet (not the
formulae) and then Edit | Paste Special | Values (check) | Multiply
(check) | OK then <Esc>. You can delete the 1.1 from the original
cell.
Some of your formulae may not work in the same way if you do this. If,
for example, you had a formula like:
=IF(A1<10,do_something,do_something_else)
and A1 used to contain 9.5 so that the do_something was actioned, then
increasing A1 by 10% will cause the do_something_else to be actioned.
Hope this helps.
Pete