more tranny trouble

My 91 celica gt no longer wants to shift out of 1st. I had too much tranny fluid in it (but I didn't drive far) so I drained about a qt and still no shifting. Car goes into park, reverse, etc no problem just won't shift over while driving. Someone suggested that I may have a pressure release valve stuck. Im unsure as what my next step should be or if this is easy or hard. Any advice would be helpful as this is unfamilar to me. Thanks!