Search for "sometext" using the exact field match operator == (two equal signs in a row). The resulting found set should only contain records in which the field contains precisely "sometext". Now select (ie put your cursor in) the field in question. Change it to the value you want all records in your found set to have. Now go to the records menu and choose "Replace Field Contents" (it's near the bottom). A menu will appear. I believe the first of the three options will accomplish what you're trying to do. I'm assuming this can't be undone so be very careful (and make a back up).