How to Change Breaking point for tablets

Hi there,
How can i change the breaking piont for tablets at the Flex columns .
I have 3 flexcolumns leftside in a 2 gridcontainer and the breakingpoint is set on 989px at tablets.
That’s for landscape vieuw not enough, I want to put it n 1280px.
With the mobile menu the css works, just not with the flexcolumns.
(At portrait vieuw and at mobile phones the Breaking pooints changes and the columns are set among themselves)
I’ve put this css:
@media only screen and (max-width: 1280px) {
.responsive #top #wrap_all .flex_column.av-break-at-tablet, .responsive #top #wrap_all .av-break-at-tablet .flex_cell {
margin: 0;
margin-bottom: 0;
width: 100%;
display: block;
}
}
.responsive #top #wrap_all .flex_column.av-break-at-tablet, .responsive #top #wrap_all .av-break-at-tablet .flex_cell {
margin: 0px 0px 20px;
width: 100%;
display: block;
}

Thanks a lot for your help
Ingrid