Here is my piece so far, as you can see in green, the melody is highlighted. It starts on beat one, but I need a write the repeat in a clearer way so that it starts on beat one the second time it is played. I presume that the melody I have written doesn't fit with the 3/4 time signature. Basically I want the start of the repeated melody (Bar 7 beat 3) to start on beat one by only changing the time signature and not the melody. Is there a way? Thanks
Yes, you can change time signatures any time you want. There's a bit of subjective decision-making involved: If what is currently m. 7 beat 3 is to become a downbeat, then what comes before? The easiest option, which I'd probably go for, is to put a 5/4 time signature at the start of m. 6, which takes up all the space until the downbeat you want.
It's then up to the performer to think about how the five beats inside that measure are grouped, but "2 + 3" seems likely. (That matters only insofar as they're feeling or communicating any "stronger beats" within that group of five.)
You could also do a few other options, like
OR 
... depending on how you feel the pulses in that section.
But a larger answer is that maybe you shouldn't worry too much about it. Plenty of compositions contain moments and rhythms that "contradict" the strong/weak structure of the meter, and you don't have to change time signature every time you do it. If you have a momentary bit of metric subversion, but everything lines up back into 3/4 when it's over, you could just leave it be and make your wishes clear through emphasis markings, or even leave it to the performer to notice how the material repeats the phrase.
