The second of your statements is true; elected representatives are unable to amend the constitution unilaterally. An amendment to the constitution has to be approved by more than 75% of representatives, and in certain cases must also be subsequently confirmed in a nation-wide referendum, and be supported by more than half of those eligible to vote - see section 436 of the 2008 Myanmar Constitution.
As the constitution also guarantees that 25% of seats in both the lower and upper houses (Pyithu Hluttaw and Amyotha Hluttaw) are controlled by the military, in sections 109(b) and 141(b), the military has a veto over constitutional change - an amendment to the constitution cannot go ahead without the votes of at least some of the unelected Defence
Services personnel serving in the legislature.
For a recent example of this power, consider the vote against the amendments proposed in 2015, which would have lowered the threshold of representatives required to pass a constitutional amendment to 70%, as well as altering the qualifications for President which currently block Aung San Suu Kyi from that role. When the proposals were defeated, San Suu Kyi had the following response:
"I am not surprised with the result," Suu Kyi told reporters after the
vote. "This makes it very clear that the constitution can never be
changed if the military representatives are opposed." She said she
didn't see the vote as a loss, since the result had been anticipated,
so her supporters should not lose hope.