bind

Note you have a typo: 2 closing component tags. In any case, the following should work

<cfform name="fform">

<!--- Set bindOnload to yes, making the bind to take effect the moment the page is loaded. --->

<cfinput type="datefield"  name="txt1" bind="cfc:app.home.cfc.bu.getFinalDate({txt2})" bindOnload="yes">

<cfinput type="datefield" name="txt2">

</cfform>

bu.cfc

<cfcomponent>

<cffunction name="getFinalDate" access="remote" returntype="string">

        <cfargument name="dateArg">

        <!--- If argument undefined or not a date, then default to date of today --->

        <cfif (not isDefined("arguments.dateArg")) or (not isDate(arguments.dateArg))>

            <cfset var dt = now()>

        <cfelse>

            <cfset var dt = parseDateTime(arguments.dateArg)>

        </cfif>

        <!--- Add 5 days--->

        <cfset var updatedDt = dateAdd("d",5,dt)>

        <!--- Format --->

        <cfset var formattedDt = dateformat(updatedDt, "mm/dd/yyyy")>

         <cfreturn formattedDt>

</cffunction>

</cfcomponent>

In your cfc, use parsedate to convert the text to a date, then use dateadd to add the five days and dateformat to convert it back to a string.