When setting up a campaign the default value of a donation on the Features tab of the campaign is set as £3.
Contact Support and reference this article.  to request a different default value from the list available.
In your case notes include the name of the Keyword and the default value that you would like to be applied instead.

Important Note:
Once the default value for the Keyword has been changed it will remain at that changed value so, for example, if changed to £5 it will remain at £5 unless request received to change the value again.
Therefore, if you intend re-using the Keyword for a different campaign when the current campaign has ended, you should repeat the above steps to supply the new value to be set.

See When does support end for Blackbaud Mobile Text Fundraising