Creating and updating table using function parameter reference

I want to create and update two tables in a function such as below, but using parameters as tablename is not allowed and gives an error. Is there any way I could achieve this?

CREATE OR REPLACE FUNCTION test ( t1  text,t2 text  ) RETURNS numeric AS $$
declare temp1 text;
declare temp2 text;
begin
    temp1=t1;
    temp2=t2;
select
product,
(case when sum(pd) <> 0 then sum(gd)/sum(pd)*100 else 0 end  ) as gppp
into temp2 from temp1  as dummy
group by dummy.product,dummy.totalclaimsgroup,dummy.avgmems,dummy.months;

update temp1 as t  set
 GPPP=(select gppp  from temp2  as dummy where dummy.product=t.product),

end
$$ LANGUAGE plpgsql


----------------------
ERROR:  syntax error at or near "$1"
LINE 1: ...en sum(gd)/sum(pd)*100 else 0 end ) as gppp from  $1  as dum...
                                                             ^
QUERY:  select product, (case when sum(pd) <> 0 then sum(gd)/sum(pd)*100 else 0 end ) as gppp from  $1  as dummy group by dummy.product,dummy.totalclaimsgroup,dummy.avgmems,dummy.months
CONTEXT:  SQL statement in PL/PgSQL function "test" near line 10

********** Error **********

ERROR: syntax error at or near "$1"
SQL state: 42601
Context: SQL statement in PL/PgSQL function "test" near line 10