Can i fix retainer that won't fit properly?

It depends: Your orthodontist should check if the retainer does not fit due to shifting of the teeth, or due to normal wear. Loose retainer may need adjustment/tightening. If your teeth shifted, new/active retainer need to be made to move teeth back into proper alignment. Do not try to adjust your retainer at home since it may cause teeth to shift.

You probably can't: See your orthodontist. It is likely that your teeth have moved due to inconsistent retainer wear, unless you have stepped on your retainer.