This function allows you to modify a group of records in the same or similar way. Pressing Alt-M brings up a field editor similar to that used to enter a new record but in this case each field specifies a replacement value for all selected records (selected records are the records that match the current restriction criteria).
The value entered against each field can be any valid expression, allowing the result to be the result of a calculation on other fields in the record. Fields for which the value is left blank won't be changed. To change a field to an empty string the null string can be specified with a pair of double quotes, ie ""
Once the field value(s) have been entered,
pressing F10 will start BTC modifying the records (after a confirmation).