I need to turn it to an esm file because I want do do several other plugins for it
thanks

http://www.newvegasnexus.com/downloads/file.php?id=34703 has what you need to change the esps to esms and back, all you need do after altering the type in the file header is to change the file extension to esm, or vice versa.

How do I do that?

Select the "File Header" in the left panel, then mouse over the empty (if it's an esp) space to the right of "Record Flags" and hit F2 for a drop-down box. From there tick "ESM", close, save, change file extension.

I can convert it to an ESM then back to an ESP for changes right?

Indeed you can In fact, if you want to edit the ESM, just untick its ESM flag and open it as a false flag ESM in the GECK and you'll be able to directly edit it. When you save, the GECK will retick the ESM flag.

Note: If your plugin overrides CELL and/or WRLD children, untick/retick the ESM flag in FNVEdit that an ONAM list will be generated as the GECK doesn't do that when it ticks the ESM flag.