Why do you need this? DNA sequences are extracted from the chromatogram. And the chromatogram is error prone is most of the cases. So DNA sequence is the fine extraction from the chromatogram.

I have 30 sequences, there are several errors in 3, I and to edit them, but when I edit (to replace "N" with "-" I using bioeitd in the chrmatogram, is dosent change. My institute dosent accept "n"s they ask for -

 The DNA should be sequenced from both end 5'&3'. then you have to reverse one sequence. then superimpose/align the sequences to find the errors. there is no way to go back to chromatogram from sequence if it is not available to the owner.

You will be able to save .txt file as fasta (or several other extensions) but I am not familiar with a software that will create ABI files from the text file. Also I don't see any reason of doing that as for analysis you need four letter bases, not the colored peaks. ABI files are binary files that are generated by ABI sequencer (software) and are generally referred to as chromatograms, electropherograms or DNA trace files - and can be viewed graphically by a ABI viewer. They are pieces of raw data and are shown in colored peaks with the associated bases (A,C,G,T or ambiguity "N") and are the source of DNA sequence. The bases are editable but the colored lines (peaks of chromatograms) are not.

You can try this online tool http://sequenceconversion.bugaco.com/converter/biology/sequences/ Hope this helps.

You can easily edit a chromatogram in Snapgene.

Alternatively, you can use 4Peaks by nucleobyte.com for sequence editing in a chromatogram.

I think you dont need to convert from txt to abi.