The answer is both yes and no.
The 'boiler plate' document clauses cannot be changed. However we have an 'additional clauses' feature where you can add or amend a clause. These appear at the end. For example an additional clause could read something like;
"Clause [number of clause] above is amended to provide that [here put what you want the change to be]"
Before doing this you should read the tenancy agreement properly as you may find the issue which concerns you is already covered. Our tenancy agreements are quite detailed and have also had input from our members who over the years have suggested various excellent clauses which we have incorporated.
You also need to be very careful about amending legal documents if you are not experienced in legal drafting - as you can make things worse. For example as discussed in our article on Unfair Terms here.
If you want to add a new clause it may be best to ask as question on the forum first. I will often draft up clauses for members to use when asked.
There are also a number of pre-drafted clauses (such as a break clause, etc) which you will find at the end of the Tenancy Agreements page.