[GENERAL] Best way to alter a foreign constraint

Some of my tables were badly designed and have 2 indexes, like the following example (lots of tables have same problem):

<<<
postgres=# \d test1
     Table "public.test1"
 Column |  Type   | Modifiers
--------+---------+-----------
 t1     | integer | not null
Indexes:
    "test1_pkey" PRIMARY KEY, btree (t1)
    "test1_t1_key" UNIQUE CONSTRAINT, btree (t1)
Referenced by:
    TABLE "test2" CONSTRAINT "test2_t1_fkey" FOREIGN KEY (t1) REFERENCES test1(t1)

postgres=# \d test2
     Table "public.test2"
 Column |  Type   | Modifiers
--------+---------+-----------
 t2     | integer | not null
 t1     | integer |
Indexes:
    "test2_pkey" PRIMARY KEY, btree (t2)
Foreign-key constraints:
    "test2_t1_fkey" FOREIGN KEY (t1) REFERENCES test1(t1)
>>>