CondaValueError: Value error: prefix already exists:

You can overwrite the existing enviroment by adding the --force option.

So:

conda create -n ENV1 python=2.7.12 anaconda --force

Make sure that you have updated you anaconda because it is a recent function.

Edit: --force feature was added in conda 4.6, but apparently on latest versions 4.7, 4.8, it doesn't always work

You can just deactivate the existing one and remove it and then create the new one.

    conda deactivate
    conda env remove -n env_name

When the conda environment was previously removed, but the actual directory still exists (for some reason), then the "conda env remove -n ENV1" will do nothing:

$ conda-env list

  # conda environments:
  #
  base                  *  /home/nmanos/miniconda
  test-env                 /home/nmanos/miniconda/envs/test-env

$ conda-env remove -n ENV1
# Nothing was removed (exit code zero)   

$ ls /home/nmanos/miniconda/envs/ENV1  
  bin  conda-meta  etc  go
# Directory still exists

So you can remove the actual ENV1 directory, as follow:

$ ENV_BASE=$(conda-env list | awk '/base/ { print $NF }')
$ echo $ENV_BASE
  /home/nmanos/miniconda

$ rm -rf "$ENV_BASE/envs/ENV1"

I simply deleted folder C:\Program Files\Miniconda2\envs\ENV1\.