Can a PhD student accused of plagiarism revise/resubmit the thesis?

I think it's relevant what kind of plagiarism this is.

If it's serious plagiarism, such as quoting large blocks of text without proper attribution, then that can be a serious academic offense. Asking why you can't just change it and re-submit is like asking why you can't just re-take an exam if you're caught cheating or why you can't just give the item back and forget everything if you're caught stealing.

Depending on your institutions procedures you might be allowed to resubmit if it's found that the plagiarism was minor, accidental or not actually plagiarism but I suspect you're missing the point.

Plagiarism in a submitted thesis isn't like a simple technical error to be corrected, it's potential wrong-doing or dishonesty to be investigated.

There are a handful of issues here. First, an accusation of plagiarism does not make it plagarism. There is most likely a formal policy for cases like this. Things get messy if the work has already been graded and accepted. Many university polices allow retroactive punishments and universities can even revoke degrees. If, after the procedure concludes, and possibly your appeals have been exhausted, you are found to have plagarised, a penalty will be applied.

That penalty can vary substantially from a warning to expulsion. The academic misconduct panel I served on took an exceptionally harsh view of academic misconduct by graduate students and a exceptionally lenient view towards undergrads. Our university policy would not allow us to expel a student on a first offense. In the case of a thesis, we would be allowed to apply 3 penalties. The most lenient we could be would be to evaluate the thesis with the plagiarized material removed. If this caused the student to fail, they would be allowed to revise the thesis. The most harsh we could be would be to require them to submit a new thesis with entirely new work. This means an entirely new thesis project with all new writing, data collection, and analysis. In the final case, for localized plagarism, we could require the student to generate a new chapter or literature review.

My advice to you is to talk to student services ASAP about the university rules and getting independent representation. Most universities prohibit using lawyers in the process. Additionally you need to know why the accusations have arisen. Did you plagarise? Did you not understand what plagarism was? Did you make a copy and paste error? Maybe it is you who have been plagarised. Finally, setup a meeting with your advisor and another with your department chair. You need to know if your advisor and/or department are backing you or not.

As a senior university academic, I would say that there is not really enough detail of the alleged misdemeanour to judge the severity of the case. However, if the fundamental research is unique and valid (as the successful defence of the thesis would imply it was), and a robust understanding of the area was demonstrated, then resubmission with ironing-out of over-similar sections would seem an appropriate outcome. If this is at a UK university, approach your Students' Union for support, they will understand the norms and Regulations and have a mandate to represent you.