You can use REPLACE function as well.
declare @STR as table( val varchar(20))
insert @STR
select '12-FLA-2-02'
declare @update varchar(20)='11111'
select * from @STR
update @STR set val=replace(val,LEFT(val, CHARINDEX('-', val)-1) ,@update)
select * from @STR
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I am just an another naive wannabe DBA trying to learn SQL Server